Rigidity is one of the oldest psychological constructs, with systematic research dating back to the late 19th century. The authors review this research in an attempt to clarify the construct of rigidity and to investigate its correlates. Rigidity is described as a multidimensional construct encompassing the tendency to form and perseverate in the use of mental and behavioral sets. A series of meta-analyses was performed based on three measures of behavioral rigidity: the Einstellung Water-Jar Task, the Wisconsin Card Sorting Task, and the motor-cognitive dimension of the Test of Behavioral Rigidity. The results indicated that rigidity is curvilinearly related to age, positively related to authoritarianism (particularly under stressful situations), and negatively related to intelligence; that men are more rigid than women; that obsessive-compulsiveness is positively related to rigidity; and that schizophrenics are more rigid than nonschizophrenic siblings and normal controls. Unresolved issues and gaps in the research are discussed.