Infrared examination techniques can aid in the detection of underdrawing. Infrared reflectography can often reveal a greater quantity of underdrawing in comparison to infrared photography. Methods of preparing reflectograms, and their proper analysis are discussed, particularly in relation to a painting attributed to Robert Campin -- the so-called Master of Flémalle -- in the Johnson Collection of the Philadelphia Museum of Art. Evidence of the artist's working methods and creative process are discussed. -- AATA