Defining a `required` field in Bootstrap

Try using required="true" in bootstrap 3

Update 2018

Since the original answer HTML5 validation is now supported in all modern browsers. Now the easiest way to make a field required is simply using the required attibute.

<input type="email" class="form-control" id="exampleInputEmail1" required>

or in compliant HTML5:

<input type="email" class="form-control" id="exampleInputEmail1" required="true">

Read more on Bootstrap 4 validation

In Bootstrap 3, you can apply a "validation state" class to the parent element: http://getbootstrap.com/css/#forms-control-validation

For example has-error will show a red border around the input. However, this will have no impact on the actual validation of the field. You'd need to add some additional client (javascript) or server logic to make the field required.

Demo: http://bootply.com/90564