Destructive and non-destructive testing are distinguished. Ultraviolet and x-ray radiography are briefly detailed, and then the article focuses on infrared techniques. Infrared photography has been in use in conservation laboratories since the late 1950's. The development and use of the videocamera and reflectograms are traced with attention to what is learned from examination of 15th and 16th century Netherlandish art, Vermeer and a 20th century American painting. Projects at the Metropolitan Museum and the National Gallery are described with statements from conservators and curators. Because Vermeer used carbon black to do his sketching, reflectography can often provide well defined images of undersketches. -- AATA