Studies on intra-individual response variability are classified into and reviewed under the rubrics of (1) variability of organic processes, (2) psychometric aspects, (3) spontaneous (aperiodic) variability, (4) systematic variability, (5) change of response with change in stimulus or situation, and (6) relationships between variability and other variables (such as brain lesions, motivation variables, and personality traits and types). The relationships between variability and neurosis or psychosis (and personality correlates in general) have not been established definitively, but it is suggested that variability may be negatively related to persistence and "character." 233-item bibliography.