Biological membranes are fascinating two-dimensional microenvironments that exhibit unique solvent behaviours due to their varying lipid composition. Although many important bioenergetic and signalling events involve the transient or permanent assembly of membrane protein complexes, the characterization of the thermodynamic and kinetic properties behind this assembly is just beginning. In particular, the molecular forces that govern protein association within these structures remain poorly understood. An understanding of the docking of transmembrane proteins to supramolecular complexes, which will make possible the development of predictive computational tools, will require detailed knowledge of interaction forces at the atomistic or residue level. Here, I review current data on supramolecular complexes in membrane environments and make a tentative comparison between assembly processes in membranes and those driven by the hydrophobic effect in water. This comparison suggests that, in addition to being controlled by specific characteristics of the lipid molecules themselves, molecular assembly in the membrane milieu also depends more generally on the entropy of the lipid fraction.