Underline in a CSS class

[Philosophaie]

How do you underline in a CSS class:

CSS file
.underline
{
underline?
}

ASP.net file

<h1 class="underline">

[DaveMaxwell]

text-decoration: underline

[StevenHu]

CSS file:

.underline
{
text-decoration: underline;
}

ASP.net file:

<h1 class="underline">