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. The features that are of greatest significance in organic chemistry are nonbonded interactions, both repulsive and attractive, and destabilization arising from distortion of bond lengths and angles from optimum values. A molecule will adopt the minimum-energy geometry available to it by rotation about single bonds. The various geometries a given molecule can attain by bond 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 quantum mechanical viewpoint.