Broadly conceived, reliability involves quantifying the consistencies and inconsistencies in observed scores. Generalizability theory, or G theory, is particularly well suited to addressing such matters in that it enables an investigator to quantify and distinguish the sources of inconsistencies in observed scores that arise, or could arise, over replications of a measurement procedure. Classical test theory is an historical predecessor to G theory and, as such, it is sometimes called a parent of G theory. Important characteristics of both theories are considered in this article, but primary emphasis is placed on G theory. In addition, the two theories are briefly compared with item response theory. (Contains 1 table and 17 footnotes.)