We consider the problem of using molecular modeling and simulation in understanding trends in solid-fluid and solid-solid phase behavior of organic molecules as we change molecular structure. Our focus is on using molecular models that are simple enough to be studied over a wide range of conditions, but realistic enough to reveal important insights into the molecular level behavior. As examples, we review some recent work on the phase behavior of molecular models of n-alkanes and aromatic molecules. For the alkanes we discuss the molecular origins of the chain length dependence of the melting points as well as the role of rotator phases in the phase diagrams of longer chain molecules. We also consider solid-fluid and solid-solid phase equilibrium in binary n-alkane mixtures. The solid-fluid equilibrium in aromatic molecules reflects a competition between the molecular shape effects and electrostatic interactions. We discuss this in the context of the solid phases of benzene and also in the context of the stability of solid phase compounds of aromatic mixtures. (c) 2004 Elsevier B.V. All rights reserved.