Sidney Felsen met Robert Rauschenberg in 1966, the same year Felsen cofounded Gemini G.E.L. (Graphic Editions Limited), a Los Angeles–based artists’ workshop and publisher of hand-printed, limited-edition lithographs. Since that time, Felsen has collaborated with many of the most important artists of the last fifty years. In addition to Rauschenberg, with whom he produced more than two hundred and fifty different prints in over thirty-five years, Felsen has worked with John Baldessari, Frank Gehry, Philip Guston, Ann Hamilton, Jasper Johns, Ellsworth Kelly, Roy Lichtenstein, Julie Mehretu, Elizabeth Murray, Bruce Nauman, Claes Oldenburg, Richard Serra, Frank Stella, and Richard Tuttle. The Artist Observed: Photographs by Sidney B. Felsen (Twin Palms Publishers, 2002) is Felsen’s firsthand documentation of working with artists at Gemini. His photographs have been exhibited at the National Gallery of Art, Washington, D.C., where several major exhibitions of Gemini's art have been held, including the exhibition The Serial Impulse at Gemini G.E.L., which opened on October 4, 2015, in celebration of the fiftieth anniversary of the print workshop.