diff --git a/db.go b/db.go
index ad9b3eb..ebddae4 100644
--- a/db.go
+++ b/db.go
@@ -1,8 +1,10 @@
 package litestream
 
 import (
+	"bytes"
 	"context"
 	"database/sql"
+	"encoding/binary"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -12,13 +14,12 @@ import (
 	"math/rand"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
 )
 
-var ErrNoGeneration = errors.New("litestream: no generation")
-
 const (
 	MetaDirSuffix = "-litestream"
 
@@ -42,6 +43,9 @@ type DB struct {
 	rtx      *sql.Tx // long running read transaction
 	pageSize int     // page size, in bytes
 
+	byteOrder    binary.ByteOrder // determined by WAL header magic
+	salt0, salt1 uint32           // read from WAL header
+
 	ctx    context.Context
 	cancel func()
 	wg     sync.WaitGroup
@@ -244,11 +248,11 @@ func (db *DB) releaseReadLock() error {
 }
 
 // CurrentGeneration returns the name of the generation saved to the "generation"
-// file in the meta data directory. Returns ErrNoGeneration if none exists.
+// file in the meta data directory. Returns empty string if none exists.
 func (db *DB) CurrentGeneration() (string, error) {
 	buf, err := ioutil.ReadFile(db.GenerationNamePath())
 	if os.IsNotExist(err) {
-		return "", ErrNoGeneration
+		return "", nil
 	} else if err != nil {
 		return "", err
 	}
@@ -257,7 +261,7 @@ func (db *DB) CurrentGeneration() (string, error) {
 
 	generation := strings.TrimSpace(string(buf))
 	if len(generation) != GenerationNameLen {
-		return "", ErrNoGeneration
+		return "", nil
 	}
 	return generation, nil
 }
@@ -277,8 +281,8 @@ func (db *DB) createGeneration() (string, error) {
 		return "", err
 	}
 
-	// Copy to shadow WAL.
-	if err := db.copyInitialWAL(generation); err != nil {
+	// Initialize shadow WAL with copy of header.
+	if err := db.initShadowWALFile(db.ShadowWALPath(generation, 0)); err != nil {
 		return "", fmt.Errorf("copy initial wal: %w", err)
 	}
 
@@ -300,42 +304,12 @@ func (db *DB) createGeneration() (string, error) {
 	return generation, nil
 }
 
-// copyInitialWAL copies the full WAL file to the initial shadow WAL path.
-func (db *DB) copyInitialWAL(generation string) error {
-	shadowWALPath := db.ShadowWALPath(generation, 0)
-	if err := os.MkdirAll(filepath.Dir(shadowWALPath), 0700); err != nil {
-		return err
-	}
-
-	// Open the initial shadow WAL file for writing.
-	w, err := os.OpenFile(shadowWALPath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
-	if err != nil {
-		return err
-	}
-	defer w.Close()
-
-	// Open the database's WAL file for reading.
-	r, err := os.Open(db.WALPath())
-	if err != nil {
-		return err
-	}
-	defer r.Close()
-
-	// Copy & sync.
-	if _, err := io.Copy(w, r); err != nil {
-		return err
-	} else if err := w.Sync(); err != nil {
-		return err
-	} else if err := w.Close(); err != nil {
-		return err
-	}
-	return nil
-}
-
 // Sync copies pending data from the WAL to the shadow WAL.
 func (db *DB) Sync() (err error) {
 	// TODO: Lock DB while syncing?
 
+	// TODO: Force "-wal" file if it doesn't exist.
+
 	// Start a transaction. This will be promoted immediately after.
 	tx, err := db.db.Begin()
 	if err != nil {
@@ -362,18 +336,29 @@ func (db *DB) Sync() (err error) {
 		return fmt.Errorf("disable autocheckpoint: %w", err)
 	}
 
-	// Look up existing generation or start a new one.
-	generation, err := db.CurrentGeneration()
-	if err == ErrNoGeneration {
-		if generation, err = db.createGeneration(); err != nil {
+	// Verify our last sync matches the current state of the WAL.
+	// This ensures that we have an existing generation & that the last sync
+	// position of the real WAL hasn't been overwritten by another process.
+	//
+	// If we are unable to verify the WAL state then we start a new generation.
+	info, err := db.verifyWAL()
+	if err != nil {
+		return fmt.Errorf("cannot verify wal state: %w", err)
+	} else if info.reason != "" {
+		// Start new generation & notify user via log message.
+		if info.generation, err = db.createGeneration(); err != nil {
 			return fmt.Errorf("create generation: %w", err)
 		}
-	} else if err != nil {
-		return fmt.Errorf("cannot find current generation: %w", err)
+		log.Printf("%s: new generation %q, %s", db.path, info.generation, info.reason)
+
+		// Clear shadow wal info.
+		info.shadowWALPath, info.shadowWALSize = "", 0
+		info.restart = false
+		info.reason = ""
 	}
 
 	// Synchronize real WAL with current shadow WAL.
-	newWALSize, err := db.syncWAL(generation)
+	newWALSize, err := db.syncWAL(info)
 	if err != nil {
 		return fmt.Errorf("sync wal: %w", err)
 	}
@@ -402,6 +387,245 @@ func (db *DB) Sync() (err error) {
 	return nil
 }
 
+// verifyWAL ensures the current shadow WAL state matches where it left off from
+// the real WAL. Returns generation & WAL sync information. If info.reason is
+// not blank, verification failed and a new generation should be started.
+func (db *DB) verifyWAL() (info syncInfo, err error) {
+	// Look up existing generation.
+	generation, err := db.CurrentGeneration()
+	if err != nil {
+		return info, fmt.Errorf("cannot find current generation: %w", err)
+	} else if generation == "" {
+		info.reason = "no generation"
+		return info, nil
+	}
+	info.generation = generation
+
+	// Determine total bytes of real WAL.
+	fi, err := os.Stat(db.WALPath())
+	if err != nil {
+		return info, err
+	}
+	info.walSize = fi.Size()
+
+	// Open shadow WAL to copy append to.
+	info.shadowWALPath, err = db.CurrentShadowWALPath(info.generation)
+	if info.shadowWALPath == "" {
+		info.reason = "no shadow wal"
+		return info, nil
+	} else if err != nil {
+		return info, fmt.Errorf("cannot determine shadow WAL: %w", err)
+	}
+
+	// Determine shadow WAL current size.
+	fi, err = os.Stat(info.shadowWALPath)
+	if err != nil {
+		return info, err
+	}
+	info.shadowWALSize = fi.Size()
+
+	// TODO: Truncate shadow WAL if there is a partial page.
+
+	// If shadow WAL is larger than real WAL then the WAL has been truncated
+	// so we cannot determine our last state.
+	if info.shadowWALSize > info.walSize {
+		info.reason = "wal truncated by another process"
+		return info, nil
+	}
+
+	// Compare WAL headers. If mismatched then the real WAL has been restarted.
+	// We'll need to start a new shadow WAL during the sync.
+	if hdr0, err := readWALHeader(db.WALPath()); err != nil {
+		return info, fmt.Errorf("cannot read wal header: %w", err)
+	} else if hdr1, err := readWALHeader(info.shadowWALPath); err != nil {
+		return info, fmt.Errorf("cannot read shadow wal header: %w", err)
+	} else {
+		info.restart = !bytes.Equal(hdr0, hdr1)
+	}
+
+	// Verify last page synced still matches.
+	offset := info.shadowWALSize - int64(db.pageSize+WALFrameHeaderSize)
+	if buf0, err := readFileAt(db.WALPath(), offset, int64(db.pageSize+WALFrameHeaderSize)); err != nil {
+		return info, fmt.Errorf("cannot read last synced wal page: %w", err)
+	} else if buf1, err := readFileAt(info.shadowWALPath, offset, int64(db.pageSize+WALFrameHeaderSize)); err != nil {
+		return info, fmt.Errorf("cannot read last synced shadow wal page: %w", err)
+	} else if !bytes.Equal(buf0, buf1) {
+		info.reason = "wal overwritten by another process"
+		return info, nil
+	}
+
+	return info, nil
+}
+
+type syncInfo struct {
+	generation    string // generation name
+	walSize       int64  // size of real WAL file
+	shadowWALPath string // name of last shadow WAL file
+	shadowWALSize int64  // size of last shadow WAL file
+	restart       bool   // if true, real WAL header does not match shadow WAL
+	reason        string // if non-blank, reason for sync failure
+}
+
+// syncWAL copies pending bytes from the real WAL to the shadow WAL.
+func (db *DB) syncWAL(info syncInfo) (newSize int64, err error) {
+	// Copy WAL starting from end of shadow WAL. Exit if no new shadow WAL needed.
+	newSize, err = db.copyToShadowWAL(info.shadowWALPath)
+	if err != nil {
+		return newSize, fmt.Errorf("cannot copy to shadow wal: %w", err)
+	} else if !info.restart {
+		return newSize, nil // If no restart required, exit.
+	}
+
+	// Parse index of current shadow WAL file.
+	dir, base := filepath.Split(info.shadowWALPath)
+	index, err := ParseWALFilename(base)
+	if err != nil {
+		return 0, fmt.Errorf("cannot parse shadow wal filename: %s", base)
+	}
+
+	// Start a new shadow WAL file with next index.
+	newShadowWALPath := filepath.Join(dir, FormatWALFilename(index+1))
+	if err := db.initShadowWALFile(newShadowWALPath); err != nil {
+		return 0, fmt.Errorf("cannot init shadow wal file: name=%s err=%w", newShadowWALPath, err)
+	}
+
+	// Copy rest of valid WAL to new shadow WAL.
+	newSize, err = db.copyToShadowWAL(newShadowWALPath)
+	if err != nil {
+		return 0, fmt.Errorf("cannot copy to new shadow wal: %w", err)
+	}
+	return newSize, nil
+}
+
+func (db *DB) initShadowWALFile(filename string) error {
+	hdr, err := readWALHeader(filename)
+	if err != nil {
+		return fmt.Errorf("read header: %w", err)
+	}
+
+	// Determine byte order for checksumming from header magic.
+	magic := binary.BigEndian.Uint32(hdr[0:])
+	switch magic {
+	case 0x377f0682:
+		db.byteOrder = binary.LittleEndian
+	case 0x377f0683:
+		db.byteOrder = binary.BigEndian
+	default:
+		return fmt.Errorf("invalid wal header magic: %x", magic)
+	}
+
+	// Read header salt.
+	db.salt0 = binary.BigEndian.Uint32(hdr[16:])
+	db.salt1 = binary.BigEndian.Uint32(hdr[20:])
+
+	// Verify checksum.
+	s0 := binary.BigEndian.Uint32(hdr[24:])
+	s1 := binary.BigEndian.Uint32(hdr[28:])
+	if v0, v1 := Checksum(db.byteOrder, 0, 0, hdr[:24]); v0 != s0 || v1 != s1 {
+		return fmt.Errorf("invalid header checksum: (%x,%x) != (%x,%x)", v0, v1, s0, s1)
+	}
+
+	// Write header to new WAL shadow file.
+	return ioutil.WriteFile(filename, hdr, 0600)
+}
+
+func (db *DB) copyToShadowWAL(filename string) (newSize int64, err error) {
+	r, err := os.Open(db.WALPath())
+	if err != nil {
+		return 0, err
+	}
+	defer r.Close()
+
+	w, err := os.OpenFile(filename, os.O_RDWR, 0600)
+	if err != nil {
+		return 0, err
+	}
+	defer w.Close()
+
+	fi, err := w.Stat()
+	if err != nil {
+		return 0, err
+	}
+
+	// Read previous checksum.
+	chksum0, chksum1, err := readLastChecksumFrom(w, db.pageSize)
+	if err != nil {
+		return 0, fmt.Errorf("last checksum: %w", err)
+	}
+
+	// Seek to correct position on both files.
+	if _, err := r.Seek(fi.Size(), io.SeekStart); err != nil {
+		return 0, fmt.Errorf("wal seek: %w", err)
+	} else if _, err := w.Seek(fi.Size(), io.SeekStart); err != nil {
+		return 0, fmt.Errorf("shadow wal seek: %w", err)
+	}
+
+	// TODO: Optimize to use bufio on reader & writer to minimize syscalls.
+
+	// Loop over each page, verify checksum, & copy to writer.
+	newSize = fi.Size()
+	buf := make([]byte, db.pageSize+WALFrameHeaderSize)
+	for {
+		// Read next page from WAL file.
+		if _, err := io.ReadFull(r, buf); err == io.EOF || err == io.ErrUnexpectedEOF {
+			break // end of file or partial page
+		} else if err != nil {
+			return newSize, fmt.Errorf("read wal: %w", err)
+		}
+
+		// Read frame salt & compare to header salt. Stop reading on mismatch.
+		salt0 := binary.BigEndian.Uint32(buf[8:])
+		salt1 := binary.BigEndian.Uint32(buf[12:])
+		if salt0 != db.salt0 || salt1 != db.salt1 {
+			break
+		}
+
+		// Verify checksum of page is valid.
+		fchksum0 := binary.BigEndian.Uint32(buf[16:])
+		fchksum1 := binary.BigEndian.Uint32(buf[20:])
+		chksum0, chksum1 = Checksum(db.byteOrder, chksum0, chksum1, buf[:8])  // frame header
+		chksum0, chksum1 = Checksum(db.byteOrder, chksum0, chksum1, buf[24:]) // frame data
+		if chksum0 != fchksum0 || chksum1 != fchksum1 {
+			return newSize, fmt.Errorf("checksum mismatch: offset=%d (%x,%x) != (%x,%x)", newSize, chksum0, chksum1, fchksum0, fchksum1)
+		}
+
+		// Add page to the new size of the shadow WAL.
+		newSize += int64(len(buf))
+	}
+
+	// Sync & close writer.
+	if err := w.Sync(); err != nil {
+		return newSize, err
+	} else if err := w.Close(); err != nil {
+		return newSize, err
+	}
+
+	return newSize, nil
+}
+
+const WALHeaderChecksumOffset = 24
+const WALFrameHeaderChecksumOffset = 16
+
+func readLastChecksumFrom(f *os.File, pageSize int) (uint32, uint32, error) {
+	// Determine the byte offset of the checksum for the header (if no pages
+	// exist) or for the last page (if at least one page exists).
+	offset := int64(WALHeaderChecksumOffset)
+	if fi, err := f.Stat(); err != nil {
+		return 0, 0, err
+	} else if fi.Size() > WALHeaderSize {
+		offset = fi.Size() - int64(pageSize) - WALFrameHeaderSize + WALFrameHeaderChecksumOffset
+	}
+
+	// Read big endian checksum.
+	b := make([]byte, 8)
+	if n, err := f.ReadAt(b, offset); err != nil {
+		return 0, 0, err
+	} else if n != len(b) {
+		return 0, 0, io.ErrUnexpectedEOF
+	}
+	return binary.BigEndian.Uint32(b[0:]), binary.BigEndian.Uint32(b[4:]), nil
+}
+
 // checkpoint performs a checkpoint on the WAL file.
 func (db *DB) checkpoint(force bool) error {
 	// Ensure the read lock has been removed before issuing a checkpoint.
@@ -433,72 +657,6 @@ func (db *DB) checkpoint(force bool) error {
 	return nil
 }
 
-// syncWAL copies pending bytes from the real WAL to the shadow WAL.
-func (db *DB) syncWAL(generation string) (newSize int64, err error) {
-	// Determine total bytes of real WAL.
-	fi, err := os.Stat(db.WALPath())
-	if err != nil {
-		return 0, err
-	}
-	walSize := fi.Size()
-
-	// Open shadow WAL to copy append to.
-	shadowWALPath, err := db.CurrentShadowWALPath(generation)
-	if err != nil {
-		return 0, fmt.Errorf("cannot determine shadow WAL: %w", err)
-	}
-
-	// TODO: Compare WAL headers.
-
-	// Determine shadow WAL current size.
-	fi, err = os.Stat(shadowWALPath)
-	if err != nil {
-		return 0, err
-	}
-	shadowWALSize := fi.Size()
-
-	// Ensure we have pending bytes to write.
-	// TODO: Verify pending bytes is divisble by (pageSize+headerSize)?
-	pendingN := walSize - shadowWALSize
-	if pendingN < 0 {
-		panic("shadow wal larger than real wal") // TODO: Handle gracefully
-	} else if pendingN == 0 {
-		return shadowWALSize, nil // wals match, exit
-	}
-
-	// TODO: Verify last page copied matches.
-
-	// Open handles for the shadow WAL & real WAL.
-	w, err := os.OpenFile(shadowWALPath, os.O_RDWR, 0600)
-	if err != nil {
-		return 0, err
-	}
-	defer w.Close()
-
-	r, err := os.Open(db.WALPath())
-	if err != nil {
-		return 0, err
-	}
-	defer r.Close()
-
-	// Seek to the correct position for each file.
-	if _, err := r.Seek(shadowWALSize, io.SeekStart); err != nil {
-		return 0, fmt.Errorf("wal seek: %w", err)
-	} else if _, err := w.Seek(shadowWALSize, io.SeekStart); err != nil {
-		return 0, fmt.Errorf("shadow wal seek: %w", err)
-	}
-
-	// Copy and sync.
-	if _, err := io.CopyN(w, r, pendingN); err != nil {
-		return 0, fmt.Errorf("copy shadow wal error: %w", err)
-	} else if err := w.Sync(); err != nil {
-		return 0, fmt.Errorf("shadow wal sync: %w", err)
-	} else if err := w.Close(); err != nil {
-		return 0, fmt.Errorf("shadow wal close: %w", err)
-	}
-	return walSize, nil
-}
-
 // monitor runs in a separate goroutine and monitors the database & WAL.
 func (db *DB) monitor() {
 	ticker := time.NewTicker(db.MonitorInterval)
@@ -543,3 +701,46 @@ func rollback(tx *sql.Tx) error {
 	}
 	return nil
 }
+
+// readWALHeader returns the header read from a WAL file.
+func readWALHeader(filename string) ([]byte, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	buf := make([]byte, WALHeaderSize)
+	n, err := io.ReadFull(f, buf)
+	return buf[:n], err
+}
+
+// readFileAt reads a slice from a file.
+func readFileAt(filename string, offset, n int64) ([]byte, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	buf := make([]byte, n)
+	if n, err := f.ReadAt(buf, offset); err != nil {
+		return buf[:n], err
+	} else if n < len(buf) {
+		return buf[:n], io.ErrUnexpectedEOF
+	}
+	return buf, nil
+}
+
+func ParseWALFilename(name string) (index int, err error) {
+	v, err := strconv.ParseInt(strings.TrimSuffix(name, WALExt), 16, 64)
+	if err != nil {
+		return 0, fmt.Errorf("invalid wal filename: %q", name)
+	}
+	return int(v), nil
+}
+
+func FormatWALFilename(index int) string {
+	assert(index >= 0, "wal index must be non-negative")
+	return fmt.Sprintf("%016d%s", index, WALExt)
+}
diff --git a/litestream.go b/litestream.go
index 7a9bda4..d2c180e 100644
--- a/litestream.go
+++ b/litestream.go
@@ -1,12 +1,25 @@
 package litestream
 
 import (
+	"encoding/binary"
 	"encoding/hex"
 	"strings"
 
 	_ "github.com/mattn/go-sqlite3"
 )
 
+// Checksum computes a running SQLite checksum over a byte slice.
+func Checksum(bo binary.ByteOrder, s0, s1 uint32, b []byte) (uint32, uint32) {
+	assert(len(b)%8 == 0, "misaligned checksum byte slice")
+
+	// Iterate over 8-byte units and compute checksum.
+	for i := 0; i < len(b); i += 8 {
+		s0 += bo.Uint32(b[i:]) + s1
+		s1 += bo.Uint32(b[i+4:]) + s0
+	}
+	return s0, s1
+}
+
 // HexDump returns hexdump output but with duplicate lines removed.
 func HexDump(b []byte) string {
 	const prefixN = len("00000000")