Fluorescence correlation spectroscopy (FCS) is a technique that allows for an extremely sensitive determination of molecular diffusion properties, down to the level of single molecules. It thus provides an attractive alternative to FRAP, requiring much less laser power and lower concentrations of fluorophores. FCS has recently been applied on live cells, and in comparison on domain-forming model membrane systems, to systematically study the influence of cholesterol on local membrane structure by investigating the mobility of selected lipid probes. The findings demonstrate the ability of FCS to sensitively distinguish between different local lipid structures, and emphasize the value of model systems for understanding membrane dynamics in general.