This talk will mainly focus on cooperative order transitions in lipid membranes, and on the effect of thermal fluctuations on biological function. We demonstrate the close relationship between heat capacity and the elastic constants, and how the presence of proteins influences these properties, including the formation of membrane domains and rafts. We investigate such domains in lipid membranes by atomic force microscopy, confocal microscopy, and study their influence on diffusion via the analysis of single molecule fluctuations by fluorescence correlation spectroscopy (FCS). All results are analyzed by statistical thermodynamics theory (Monte Carlo simulations).