Rembrandt prints in the collections of the National Gallery of Art, Washington DC, Boston Museum of Fine Arts, and the Metropolitan Museum, New York, were examined to determine the types of papers used to understand better the artist's working methods and to establish a watermark archive as a reference source for Rembrandt scholars. The preferred methods of recording watermarks at the National Gallery (beta radiography and other techniques such as tracing, transmitted light photography, Dylux 503, and low voltage x-radiography) are discussed. Categories developed were: 1) identical image 2) very similar image; 3) closely related image; and 4) no closely related images. Those found at the National Gallery fell into five sets of identical watermarks. Data in the National Gallery's watermark archive is grouped according to information about artist and artwork, information about degree of watermark similarity, and information about beta radiography and paper manufacture. In conjunction with this research a computerized watermark archive was developed.