How to horizontal & vertical center a div

Here is the source code:

<div id = "outer">
     <div id="top">
     ....
     </div>

     <div id="inner">
     ....
     </div>

     <div id="bottom">
     ....
     </div>
</div>

How do I make the div (id inner), horizontal & vertical center?

I can horizontal center, but I can't make a vertical center...