Empirical force field calculations (e.f.f.) provide reliable predictions of the geometrical structure of organic molecules, particularly hydrocarbons. When intermolecular interactions are combined with the intramolecular field, the influence of a condensed phase on molecular conformation can be investigated. The method can be applied rather generally to the solution and/or refinement of crystal structures and, more recently, to defining the most probable arrangements of organic molecules chemisorbed on metal surfaces. Finally, a summary is provided of how the e.f.f. technique can be used, if supplemented by calculations of lattice dynamics, in the study of phase transitions in molecular crystals.