diff --git a/spec/atom/app-spec.coffee b/spec/atom/app-spec.coffee
index 91f4da67c..61d7d01c7 100644
--- a/spec/atom/app-spec.coffee
+++ b/spec/atom/app-spec.coffee
@@ -14,30 +14,14 @@ describe "App", ->
 
   describe "open", ->
     describe "when opening a filePath", ->
-      it "displays it in a new window", ->
+      it "displays it in a new window with the contents of the file loaded", ->
         filePath = require.resolve 'fixtures/sample.txt'
         expect(app.windows().length).toBe 0
 
         app.open filePath
 
         expect(app.windows().length).toBe 1
-
-      it "loads a buffer with filePath contents", ->
-        filePath = require.resolve 'fixtures/sample.txt'
-
-        app.open filePath
-
         newWindow = app.windows()[0]
         expect(newWindow.rootView.editor.buffer.url).toEqual filePath
         expect(newWindow.rootView.editor.buffer.getText()).toEqual fs.read(filePath)
 
-    describe "when opening a dirPath", ->
-      it "loads an empty buffer", ->
-        dirPath = require.resolve 'fixtures'
-
-        app.open dirPath
-
-        newWindow = app.windows()[0]
-        expect(newWindow.rootView.editor.buffer.url).toBeUndefined
-        expect(newWindow.rootView.editor.buffer.getText()).toBe ""
-
diff --git a/spec/atom/editor-spec.coffee b/spec/atom/editor-spec.coffee
index 5e03e9f72..9f4f80611 100644
--- a/spec/atom/editor-spec.coffee
+++ b/spec/atom/editor-spec.coffee
@@ -1,3 +1,4 @@
+Buffer = require 'buffer'
 Editor = require 'editor'
 $ = require 'jquery'
 ck = require 'coffeekup'
@@ -12,7 +13,6 @@ describe "Editor", ->
   beforeEach ->
     filePath = require.resolve 'fixtures/sample.txt'
     tempFilePath = '/tmp/temp.txt'
-    spyOn(Editor.prototype.viewProperties, 'open').andCallThrough()
     editor = Editor.build()
 
   afterEach ->
@@ -29,41 +29,45 @@ describe "Editor", ->
       editor.destroy()
       expect(editor.aceEditor.destroy).toHaveBeenCalled()
 
-  describe "open(url)", ->
-    describe "when called with a url", ->
-      it "loads a buffer for the given url into the editor", ->
-        editor.open(filePath)
-        fileContents = fs.read(filePath)
-        expect(editor.getAceSession().getValue()).toBe fileContents
-        expect(editor.buffer.url).toBe(filePath)
-        expect(editor.buffer.getText()).toEqual fileContents
+  describe "setBuffer(buffer)", ->
+    it "sets the document on the aceSession", ->
+      buffer = new Buffer filePath
+      editor.setBuffer buffer
 
-      it "sets the mode on the session based on the file extension", ->
-        editor.open('something.js')
-        expect(editor.getAceSession().getMode().name).toBe 'javascript'
+      fileContents = fs.read(filePath)
+      expect(editor.getAceSession().getValue()).toBe fileContents
 
-        editor.open('something.text')
-        expect(editor.getAceSession().getMode().name).toBe 'text'
+    it "restores the ace edit session for a previously assigned buffer", ->
+      buffer = new Buffer filePath
+      editor.setBuffer buffer
 
-    describe "when called with null", ->
-      it "loads an empty buffer with no url", ->
-        editor.open()
-        expect(editor.getAceSession().getValue()).toBe ""
-        expect(editor.buffer.url).toBeUndefined()
-        expect(editor.buffer.getText()).toEqual ""
+      aceSession = editor.getAceSession()
+
+      editor.setBuffer new Buffer(tempFilePath)
+      expect(editor.getAceSession()).not.toBe(aceSession)
+
+      editor.setBuffer(buffer)
+      expect(editor.getAceSession()).toBe aceSession
+
+    it "sets the language mode based on the file extension", ->
+      buffer = new Buffer "something.js"
+      editor.setBuffer buffer
+
+      expect(editor.getAceSession().getMode().name).toBe 'javascript'
 
   describe "when the text is changed via the ace editor", ->
     it "updates the buffer text", ->
-      editor.open(filePath)
-      expect(editor.buffer.getText()).not.toMatch /^.ooo/
+      buffer = new Buffer(filePath)
+      editor.setBuffer(buffer)
+      expect(buffer.getText()).not.toMatch /^.ooo/
       editor.getAceSession().insert {row: 0, column: 1}, 'ooo'
-      expect(editor.buffer.getText()).toMatch /^.ooo/
+      expect(buffer.getText()).toMatch /^.ooo/
 
   describe "save", ->
     describe "when the current buffer has a url", ->
       beforeEach ->
-        editor.open tempFilePath
-        expect(editor.buffer.url).toBe tempFilePath
+        buffer = new Buffer(tempFilePath)
+        editor.setBuffer(buffer)
 
       it "saves the current buffer to disk", ->
         editor.buffer.setText 'Edited buffer!'
diff --git a/spec/atom/project-spec.coffee b/spec/atom/project-spec.coffee
index 39f6517a6..9d50de015 100644
--- a/spec/atom/project-spec.coffee
+++ b/spec/atom/project-spec.coffee
@@ -14,3 +14,28 @@ describe "Project", ->
         project.getFilePaths().done (result) ->
           expect(result).toEqual(expectedPaths)
 
+  describe ".open(path)", ->
+    absolutePath = null
+    beforeEach ->
+      absolutePath = require.resolve('fixtures/dir/a')
+
+    it "always returns the same buffer for the same canonical path", ->
+      buffer = project.open(absolutePath)
+      expect(project.open(absolutePath)).toBe buffer
+      expect(project.open('a')).toBe buffer
+
+    describe "when given an absolute path", ->
+      it "returns a buffer for the given path", ->
+        expect(project.open(absolutePath).url).toBe absolutePath
+
+    describe "when given a relative path", ->
+      it "returns a buffer for the given path (relative to the project root)", ->
+        expect(project.open('a').url).toBe absolutePath
+
+  describe ".resolve(path)", ->
+    it "returns an absolute path based on the project's root", ->
+      absolutePath = require.resolve('fixtures/dir/a')
+      expect(project.resolve('a')).toBe absolutePath
+      expect(project.resolve(absolutePath + '/../a')).toBe absolutePath
+      expect(project.resolve('a/../a')).toBe absolutePath
+
diff --git a/spec/atom/root-view-spec.coffee b/spec/atom/root-view-spec.coffee
index 3ba05cfba..a2d3ee5a8 100644
--- a/spec/atom/root-view-spec.coffee
+++ b/spec/atom/root-view-spec.coffee
@@ -4,11 +4,13 @@ RootView = require 'root-view'
 
 describe "RootView", ->
   rootView = null
+  project = null
   url = null
 
   beforeEach ->
     url = require.resolve 'fixtures/dir/a'
     rootView = RootView.build {url}
+    project = rootView.project
 
   describe "initialize", ->
     describe "when called with a url that references a file", ->
@@ -49,11 +51,17 @@ describe "RootView", ->
           rootView.toggleFileFinder()
           expect(rootView.find('.file-finder')).not.toExist()
 
-      it "shows all urls for the current project", ->
+      it "shows all relative file paths for the current project", ->
         waitsForPromise ->
           rootView.toggleFileFinder()
-        runs ->
-          expect(rootView.fileFinder.urlList.children('li').length).toBe 3
+
+        waitsForPromise ->
+          project.getFilePaths().done (paths) ->
+            expect(rootView.fileFinder.urlList.children('li').length).toBe paths.length
+
+            for path in paths
+              relativePath = path.replace(project.url, '')
+              expect(rootView.fileFinder.urlList.find("li:contains(#{relativePath}):not(:contains(#{project.url}))")).toExist()
 
     describe "when there is no project", ->
       beforeEach ->
@@ -65,3 +73,14 @@ describe "RootView", ->
         rootView.toggleFileFinder()
         expect(rootView.find('.file-finder')).not.toExist()
 
+  describe "when a path is selected in the file finder", ->
+    it "opens the file associated with that path in the editor", ->
+      waitsForPromise -> rootView.toggleFileFinder()
+      runs ->
+        firstLi = rootView.fileFinder.find('li:first')
+        rootView.fileFinder.select()
+        expect(rootView.editor.buffer.url).toBe(project.url + firstLi.text())
+
+
+
+
diff --git a/spec/stdlib/fs-spec.coffee b/spec/stdlib/fs-spec.coffee
index 66ce34e1b..1e43de342 100644
--- a/spec/stdlib/fs-spec.coffee
+++ b/spec/stdlib/fs-spec.coffee
@@ -11,6 +11,12 @@ describe "fs", ->
         expect(fs.directory(require.resolve('fixtures/dir'))).toBe require.resolve('fixtures/dir/')
         expect(fs.directory(require.resolve('fixtures/dir/'))).toBe require.resolve('fixtures/dir/')
 
+  describe ".join(paths...)", ->
+    it "concatenates the given paths with the directory seperator", ->
+      expect(fs.join('a')).toBe 'a'
+      expect(fs.join('a', 'b', 'c')).toBe 'a/b/c'
+      expect(fs.join('/a/b/', 'c', 'd')).toBe '/a/b/c/d'
+      expect(fs.join('a', 'b/c/', 'd/')).toBe 'a/b/c/d/'
 
   describe ".async", ->
     describe ".listFiles(directoryPath, recursive)", ->
@@ -19,15 +25,13 @@ describe "fs", ->
 
       describe "when recursive is true", ->
         it "returns a promise that resolves to the recursive contents of that directory that are files", ->
-          waitsFor (complete) ->
+          waitsForPromise ->
             fs.async.listFiles(directoryPath, true).done (result) ->
               expect(result).toEqual (path for path in fs.list(directoryPath, true) when fs.isFile(path))
-              complete()
 
       describe "when recursive is false", ->
         it "returns a promise that resolves to the contents of that directory that are files", ->
-          waitsFor (complete) ->
+          waitsForPromise ->
             fs.async.listFiles(directoryPath).done (result) ->
               expect(result).toEqual (path for path in fs.list(directoryPath) when fs.isFile(path))
-              complete()
 
diff --git a/src/atom/editor.coffee b/src/atom/editor.coffee
index 7d9a7a7dd..aafd81c5a 100644
--- a/src/atom/editor.coffee
+++ b/src/atom/editor.coffee
@@ -12,11 +12,11 @@ class Editor extends Template
   viewProperties:
     aceEditor: null
     buffer: null
-    editorElement: null
 
     initialize: () ->
+      @aceSessions = {}
       @buildAceEditor()
-      @open()
+      @setBuffer(new Buffer)
 
     shutdown: ->
       @destroy()
@@ -24,10 +24,11 @@ class Editor extends Template
     destroy: ->
       @aceEditor.destroy()
 
-    open: (url) ->
-      @buffer = new Buffer(url)
-      session = new EditSession(@buffer.aceDocument, @buffer.getMode())
-      @aceEditor.setSession(session)
+    setBuffer: (@buffer) ->
+      @aceEditor.setSession @getAceSessionForBuffer(buffer)
+
+    getAceSessionForBuffer: (buffer) ->
+      @aceSessions[@buffer.url] ?= new EditSession(@buffer.aceDocument, @buffer.getMode())
 
     buildAceEditor: ->
       @aceEditor = ace.edit this[0]
diff --git a/src/atom/project.coffee b/src/atom/project.coffee
index 87c501104..9fa61c654 100644
--- a/src/atom/project.coffee
+++ b/src/atom/project.coffee
@@ -1,9 +1,13 @@
 fs = require 'fs'
+Buffer = require 'buffer'
 
 module.exports =
 
 class Project
+  buffers: null
+
   constructor: (@url) ->
+    @buffers = {}
 
   getFilePaths: ->
     projectUrl = @url
@@ -11,3 +15,11 @@ class Project
       urls = (url.replace(projectUrl, "") for url in urls when fs.isFile(url))
       urls
 
+  open: (filePath) ->
+    filePath = @resolve filePath
+    @buffers[filePath] ?= new Buffer(filePath)
+
+  resolve: (filePath) ->
+    filePath = fs.join(@url, filePath) unless filePath[0] == '/'
+    fs.absolute filePath
+
diff --git a/src/atom/root-view.coffee b/src/atom/root-view.coffee
index 69c5ce40f..eaf4a5bcc 100644
--- a/src/atom/root-view.coffee
+++ b/src/atom/root-view.coffee
@@ -2,6 +2,7 @@ $ = require 'jquery'
 fs = require 'fs'
 
 Template = require 'template'
+Buffer = require 'buffer'
 Editor = require 'editor'
 FileFinder = require 'file-finder'
 Project = require 'project'
@@ -23,7 +24,7 @@ class RootView extends Template
 
       if url
         @project = new Project(fs.directory(url))
-        @editor.open(url) if fs.isFile(url)
+        @editor.setBuffer(@project.open(url)) if fs.isFile(url)
 
     addPane: (view) ->
       pane = $('<div class="pane">')
@@ -37,7 +38,10 @@ class RootView extends Template
         @fileFinder.remove()
         @fileFinder = null
       else
-        @project.getFilePaths().done (urls) =>
-          @fileFinder = FileFinder.build({urls, selected: (url) => @editor.open(url)})
-          @addPane(@fileFinder)
+        @project.getFilePaths().done (paths) =>
+          relativePaths = (path.replace(@project.url, "") for path in paths)
+          @fileFinder = FileFinder.build
+            urls: relativePaths
+            selected: (relativePath) => @editor.setBuffer(@project.open(relativePath))
+          @addPane @fileFinder
           @fileFinder.input.focus()
diff --git a/src/stdlib/fs.coffee b/src/stdlib/fs.coffee
index 0950232e9..614da994f 100644
--- a/src/stdlib/fs.coffee
+++ b/src/stdlib/fs.coffee
@@ -35,6 +35,11 @@ module.exports =
   exists: (path) ->
     OSX.NSFileManager.defaultManager.fileExistsAtPath_isDirectory path, null
 
+  join: (paths...) ->
+    return paths[0] if paths.length == 1
+    [first, rest...] = paths
+    first.replace(/\/?$/, "/") + @join(rest...)
+
   # Returns true if the file specified by path exists and is a
   # directory.
   isDirectory: (path) ->