Adding multiple class using ng-class

To apply different classes when different expressions evaluate to true:

<div ng-class="{class1 : expression1, class2 : expression2}">
    Hello World!
</div>

To apply multiple classes when an expression holds true:

<!-- notice expression1 used twice -->
<div ng-class="{class1 : expression1, class2 : expression1}">
    Hello World!
</div>

or quite simply:

<div ng-class="{'class1 class2' : expression1}">
    Hello World!
</div>

Notice the single quotes surrounding css classes.

For the ternary operator notation:

<div ng-class="expression1? 'class1 class2' : 'class3 class4'">

An incredibly powerful alternative to other answers here:

ng-class="[  { key: resulting-class-expression }[ key-matching-expression ], ..  ]"

Some examples:

1. Simply adds 'class1 class2 class3' to the div:

<div ng-class="[{true: 'class1'}[true], {true: 'class2 class3'}[true]]"></div>

2. Adds 'odd' or 'even' classes to div, depending on the $index:

<div ng-class="[{0:'even', 1:'odd'}[ $index % 2]]"></div>

3. Dynamically creates a class for each div based on $index

<div ng-class="[{true:'index'+$index}[true]]"></div>

If $index=5 this will result in:

<div class="index5"></div>

Here's a code sample you can run:

var app = angular.module('app', []); 
app.controller('MyCtrl', function($scope){
  $scope.items = 'abcdefg'.split('');
}); 

.odd  { background-color: #eee; }
.even { background-color: #fff; }
.index5 {background-color: #0095ff; color: white; font-weight: bold; }
* { font-family: "Courier New", Courier, monospace; }

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.0.1/angular.min.js"></script>

<div ng-app="app" ng-controller="MyCtrl">
  <div ng-repeat="item in items"
    ng-class="[{true:'index'+$index}[true], {0:'even', 1:'odd'}[ $index % 2 ]]">
    index {{$index}} = "{{item}}" ng-class="{{[{true:'index'+$index}[true], {0:'even', 1:'odd'}[ $index % 2 ]].join(' ')}}"
  </div>
</div>