Div help

I’m creating a custom theme and I’m wondering if anyone can help me with some <div>’s.

Sample code

<div id="wrapper">
    <div id="content">
       <h2>Test</h2>
       <p>This is a test</p>
    </div>
    <div id="sidebar">
       <p>Some simple text</p>
    </div>
</div>

I can’t get the heights to be equal when one has more content than the other. Can anyone shed any light on this for me?