How to center div?

Question

I have a problem with centering div in HTML (vertical & horizontal). My code looks something like this:

SOME HTML
         


        

Answer 1

#container{
    width: 366px;
    height: 274px;
    top: 50%;
    left: 50%;
    margin: -137px 0 0 -188px;
    position:absolute;
}