diff --git a/.gitignore b/.gitignore
index 034b038..9fd7ff4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -66,4 +66,7 @@ typings/
 # Ignore Terraform stuff in examples
 .terraform
 *.tfstate
-*.tfstate.backup
\ No newline at end of file
+*.tfstate.backup
+
+# WebStorm
+.idea
diff --git a/src/FileItem.ts b/src/FileItem.ts
index 651709a..6d7ce2a 100644
--- a/src/FileItem.ts
+++ b/src/FileItem.ts
@@ -1,9 +1,11 @@
 import * as Path from 'path'
 import * as fs from 'fs-extra'
 import { Item, Container } from './types'
+import * as crypto from 'crypto'
 
 export default class FileItem implements Item {
   private baseDirectory: string
+  private computedEtag?: string
 
   public key: string
   public modtime: Date
@@ -21,6 +23,30 @@ export default class FileItem implements Item {
     this.isSymbolicLink = stats.isSymbolicLink()
   }
 
+  public etag(): string {
+    if (this.computedEtag === undefined) {
+      this.computedEtag = FileItem.computeEtag(Path.join(this.baseDirectory, this.key))
+    }
+    return this.computedEtag!
+  }
+
+  private static computeEtag(filepath: string): string {
+    const hash = crypto.createHash('md5')
+    const buffer = Buffer.alloc(8192)
+
+    const fd = fs.openSync(filepath, 'r')
+    try {
+      let bytesRead
+      do {
+        bytesRead = fs.readSync(fd, buffer, 0, 8192, null)
+        hash.update(buffer.slice(0, bytesRead))
+      } while (bytesRead === 8192)
+    } finally {
+      fs.closeSync(fd)
+    }
+    return `"${hash.digest('hex')}"`
+  }
+
   read(): Promise<Buffer> {
     return fs.readFile(Path.join(this.baseDirectory, this.key))
   }
diff --git a/src/S3Container.ts b/src/S3Container.ts
index 076d62d..c54515d 100644
--- a/src/S3Container.ts
+++ b/src/S3Container.ts
@@ -93,6 +93,7 @@ export default class S3Container extends S3Prefixer implements Container {
     const s3Object: S3.Object = {
       Key: key,
       LastModified: objectHead.LastModified,
+      ETag: objectHead.ETag,
       Size: body.length,
     }
     const ret: Item = new S3Item(item.key, s3Object)
diff --git a/src/S3Item.ts b/src/S3Item.ts
index 41c06b1..880084d 100644
--- a/src/S3Item.ts
+++ b/src/S3Item.ts
@@ -7,6 +7,7 @@ const s3 = new S3()
 export default class S3Item extends S3Prefixer implements Item {
   private s3Object: S3.Object
   private bucketName: string
+  private remoteEtag?: string
 
   public key: string
   public modtime: Date
@@ -30,6 +31,7 @@ export default class S3Item extends S3Prefixer implements Item {
     }
     this.key = s3Object.Key
     this.modtime = s3Object.LastModified
+    this.remoteEtag = s3Object.ETag
     this.size = s3Object.Size
     this.isSymbolicLink = false
   }
@@ -59,4 +61,8 @@ export default class S3Item extends S3Prefixer implements Item {
   public read(): Promise<Buffer> {
     return this.getBody()
   }
+
+  public etag(): string {
+    return this.remoteEtag || ''
+  }
 }
\ No newline at end of file
diff --git a/src/Stats.ts b/src/Stats.ts
index 8ddcb1e..bdc5697 100644
--- a/src/Stats.ts
+++ b/src/Stats.ts
@@ -18,8 +18,8 @@ export interface StatsObject {
 }
 
 export default class Stats implements StatsObject {
-  public source: string
-  public target: string
+  public source: string = ''
+  public target: string = ''
   public diffs: ItemDiff[] = []
   public distributions?: CloudFront.DistributionSummary[]
   public invalidations?: string[]
diff --git a/src/Websync.ts b/src/Websync.ts
index 6d2096a..30dc2e7 100644
--- a/src/Websync.ts
+++ b/src/Websync.ts
@@ -77,12 +77,12 @@ export default class Websync extends EventEmitter implements WebsyncEmitter {
   private initialized: boolean = false
   private completed: boolean = false
 
-  private diffs: ItemDiff[]
+  private diffs: ItemDiff[] = []
   private stats: Stats
-  private transfer: Transfer
+  private transfer?: Transfer
   private invalidations: string[] | undefined
   private invalidator: CloudFrontInvalidator | undefined
-  private distributions: string[]
+  private distributions?: string[]
 
   private completeCount = 0
 
@@ -197,7 +197,9 @@ export default class Websync extends EventEmitter implements WebsyncEmitter {
     }
 
     try {
-      await this.transfer.complete()
+      if (this.transfer !== undefined) {
+        await this.transfer.complete()
+      }
     } catch (err) {
       throw new Errors.TransferFailed(err)
     }
diff --git a/src/cli.ts b/src/cli.ts
index 51030e5..f517f80 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -16,7 +16,7 @@ Options:
   --config             Provide a configuration file (js or json)
   --include            Pattern to include
   --exclude            Patter to exclude
-  --diffBy             Property by which objects are diffed (modtime or size), default is modtime
+  --diffBy             Property by which objects are diffed (etag, modtime or size), default is modtime
   --wildcardPolicy     Set the wildcard policy (majority, minority, or unanmious)
   --wildcardAll        Append wildcard to all invalidations
   --invalidateDeletes  Invalidate delete transfers
diff --git a/src/diff.ts b/src/diff.ts
index 16582be..6fb7356 100644
--- a/src/diff.ts
+++ b/src/diff.ts
@@ -1,6 +1,6 @@
 import { Container, Item, ItemDiffType, ItemDiff, FilterOptions } from './types'
 
-export type DiffKey = 'modtime' | 'size'
+export type DiffKey = 'modtime' | 'size' | 'etag'
 
 const getItem = (items: Item[], key: string): Item | undefined => {
   for (let i = 0; i < items.length; i++) {
@@ -14,6 +14,8 @@ const shouldUpdate = (source: Item, target: Item, diffKey: DiffKey): boolean =>
       return source.modtime.getTime() > target.modtime.getTime()
     case 'size':
       return source.size !== target.size
+    case 'etag':
+      return source.etag() !== target.etag()
   }
 }
 
diff --git a/src/types.ts b/src/types.ts
index 2e1ebbf..cc13dc5 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -13,6 +13,7 @@ export interface Item {
   modtime: Date
   size: number
   isSymbolicLink: boolean
+  etag(): string
   read(): Promise<Buffer>
 }
 
diff --git a/test/unit/S3Container.test.ts b/test/unit/S3Container.test.ts
index d1e8007..cb196e1 100644
--- a/test/unit/S3Container.test.ts
+++ b/test/unit/S3Container.test.ts
@@ -64,6 +64,7 @@ const createContainerTest = (prefix: string = '') => () => {
     const item: Item = {
       key: 'bang/a/b/d.txt',
       modtime: new Date(),
+      etag: () => '3e25960a79dbc69b674cd4ec67a72c62',
       size: body.length,
       isSymbolicLink: false,
       read: () => Promise.resolve(body),
@@ -89,6 +90,7 @@ const createContainerTest = (prefix: string = '') => () => {
     const item: Item = {
       key: 'test.txt',
       modtime: new Date(),
+      etag: () => '3e25960a79dbc69b674cd4ec67a72c62',
       size: body.length,
       isSymbolicLink: false,
       read: () => Promise.resolve(body),
@@ -114,6 +116,7 @@ const createContainerTest = (prefix: string = '') => () => {
     const item: Item = {
       key: 'index.html',
       modtime: new Date(),
+      etag: () => '3e25960a79dbc69b674cd4ec67a72c62',
       size: body.length,
       isSymbolicLink: false,
       read: () => Promise.resolve(body),
diff --git a/test/utils.ts b/test/utils.ts
index d377163..beaf0f7 100644
--- a/test/utils.ts
+++ b/test/utils.ts
@@ -20,6 +20,10 @@ export class MockItem implements Item {
   public read(): Promise<Buffer> {
     return Promise.resolve(new Buffer(this.size))
   }
+
+  etag(): string {
+    return faker.random.word()
+  }
 }
 
 const randomContainerType = (): ContainerType =>