Classical test theory (CTT) comprises a set of concepts and methods that provide a basis for many of the measurement tools currently used in health research. The assumptions and concepts underlying CTT are discussed. These include item and scale characteristics that derive from CTT as well as types of reliability and validity. Procedures commonly used in the development of scales under CTT are summarized, including factor analysis and the creation of scale scores. The advantages and disadvantages of CTT, its use across populations, and its continued use in the face of more recent measurement models are also discussed.