The complex and multi-faceted nature of the cognitive process of reasoning is discussed. The factor-analytic history of the isolation and definition of the three most important currently recognized reasoning factors (general reasoning, induction, and logical or deductive reasoning) is traced through the psychometric literature, and the interdependence of these abilities is discussed. Examples of many test item-types which have been found to define these factors are presented with empirical documentation of their importance and use in a variety of test batteries and with different examinee populations. (Author/BW)