Normal pre- and post-natal changes in the interorbital distance are described. Causes of illusory hypertelorism include flat nasal bridge, epicanthic folds, exotropia, widely-spaced eyebrows, narrow palpebral fissures, and dystopia canthorum. Measurements of hypertelorism may involve soft tissues or bone, and a number of indices have also been proposed. Various types of measurements are evaluated and recommendations suggested. Possible pathogenetic mechanisms for hypertelorism include: early ossification of the lesser wings of the sphenoid; failure in nasal capsule development allowing the primitive brain vesicle to protrude into the space normally occupied by the capsule resulting in morphokinetic arrest in the position of the eyes; and disturbances of the cranial base in Apert syndrome. Associations with increased interorbital distance are also discussed: orofacial clefting, nonprotruding lipomas of the corpus callosum, calcification of the falx cerebri, duplication of the crista galli, wrinkling of the nose, and tissue tags of the nose. Finally, experimental models of hypertelorism in animals are discussed.