Relatively little is known about the interactions that stabilize membrane proteins within the lipid bilayer. The bilayer is very heterogeneous, meaning its composition and physical properties vary strongly with lateral position and with depth. Experiments have shown that amino-acid side-chains usually have a strong preference for one region of the bilayer. For example, tryptophan residues strongly prefer to reside in the glycerol region of the bilayer. Using all-atom simulations, we are studying how and why tryptophan side-chains segregate in different regions of the bilayer.