Combining methods of art history and conservation science, this online scholarly catalogue examines the watercolors of James McNeill Whistler (1834–1903) in the Freer Gallery of Art. Together with the 2019 Freer exhibition and related publication Whistler in Watercolor (Freer Gallery of Art, 2019), this site presents the most comprehensive exploration of the subject yet undertaken. This project brought together art historians, a paper conservator, and a materials scientist. Their collaboration synthesized each object's exhibition history and critical reception with technical analysis of Whistler's paper, pigments, and working techniques. The results of this study illuminate the artist's working methods in the medium and establish a baseline of information to which other collections of Whistler's watercolors can be compared. This online catalogue presents the results of this research in detail. The related volume, Whistler in Watercolor expands on this research by clarifying the role watercolor played in Whistler's artistic re-invention in the 1880s. It illuminates Whistler's watercolor practice in relation to his peers in both Britain and the United States, and explains the connection between his watercolor work and his experiments in other media.