The total energy of a molecule is directly related to its geometry. Several aspects of molecular geometry can be recognized, and, to some extent, the energetic consequences can be dissected and attributed to specific structural features. Among the factors which contribute to total energy and have a recognizable connection with molecular geometry are nonbonded repulsions, ring strain in cyclic systems, and destabilization resulting from distortion of bond lengths or bond angles from optimal values. Conversely, there are stabilizing interactions which have geometric constraints. Most of these can be classed as stereoelectronic effects; that is, a particular geometric relationship is required to maximize the stabilizing interaction. In addition there are other molecular interactions, such as hydrogen bonds and dipole—dipole interactions, where the strength of the interaction will be strongly dependent on geometric factors. A molecule will adopt the minimum energy geometry that is available by rotations about single bonds. The various shapes that a given molecule can attain by these rotations are called conformations. The principles on which analysis of conformational equilibria and rotational processes are based have been developed using a classical mechanical framework, for the most part. More recently, the problem of detailed interpretation of molecular geometry has also been attacked from the molecular orbital viewpoint.