Everyone knows about blood cholesterol; there's 'the good kind' and 'the bad kind.' But the truth is that most of the cholesterol molecules that are in your body are not out floating around in your blood, they're tucked away in your cell membranes! In fact, cholesterol is the single most abundant species of molecule in mammalian cell membranes, and it seems to play an important part in modulating the physical properties of those membranes. During my talk, I'll summarize what is currently known about the different biophysical roles that cholesterol plays. Then I'll go on to describe some exciting new experiments that we have developed in order to probe the nature of cholesterol-rich, 'lipid raft' membrane domains, a subject of intense interest in membrane biophysics today.