This document discusses various methods for diagnosing dental caries. It describes the ideal requirements of a caries diagnostic test, such as being accurate, sensitive, specific, reliable and reproducible. It then examines numerous in vivo and in vitro methods for caries diagnosis, including visual examination, radiographs, fiber-optic transillumination, fluorescence techniques, and more novel methods like magnetic resonance microimaging and optical coherence tomography. Each method is described in terms of its principles, advantages, limitations and appropriate uses in diagnosing dental caries.