The generation of intense ultrashort light pulses in mode-locked laser systems has made possible a wide range of new experiments designed to study the interaction of light with matter. For the quantitative interpretation of the results, accurate measurement of the optical pulse structure is essential, and it is the purpose of this paper to review all the diagnostic techniques currently available. The recent rapid development of the electron-optical streak camera is highlighted, while considerable space is devoted to an extensive description of the many second- and higher order correlation measurements (including the popular two-photon fluorescence method). A discussion of ultrafast shutter techniques is also included, together with a section on pulse chirping and dynamic spectroscopy.