Abstract: Statistical thermodynamics simulations in recent years have considerably altered the view of biological and model membranes. Fluctuations in concentration and state, as well as in area and volume, lead to the prediction of a domain structure and anomalies in the elastic constants. New experiments support the predictions made from these models. Domains have been found and elasticities have been measured, both in model and in biological systems. The concept of hydrophobic matching, underlying the simulations, has experimentally been shown to be relevant for the function of membrane proteins. The growing interaction of theoreticians with biologists will soon lead to a wide acceptance of the importance of these theoretical concepts.