Why isn't heigh:auto working for site container

Why isn't heigh:auto working for site container

I have been developing a site using absolute height values and was curious
as to why height:auto isn't working for me, can anyone shred some light on
this.
HTML Structure
<div id="site-content">
<div id="menuconatiner"></div>
<div id="review-container"></div>
</div>
CSS
#site-content{
webkit-border-radius: 5px;
-moz-border-radius: 5px;
border-radius: 5px;
margin: 0 auto;
width: 938px;
padding-bottom:20px;
background-color:white;}
#menuconatiner{
margin:5px;
float:left;
width:190px;}