In her own quiet way, Ileana Sonnabend—dubbed the Mom of Pop art—has been as great a force in the New York art world as her wartime sweetheart, the inimitable dealer Leo Castelli, who gave Jasper Johns his first show. But it was Sonnabend who, in her Paris gallery, introduced the American Pop artists to Europe. And in the early eighties, it was she who alerted New York to a new wave of German painting, the neo-Expressionists Georg Baselitz and A. R. Penck. Now, the private collection she assembled with her late partner and husband, Michael, goes public in a dazzling exhibition that spans nearly half a century of contemporary art. The pieces, by more than 45 artists, range from once-startling, now blue-chip classics by Johns, Rauschenberg, Warhol, and Gilbert & George to Christian Boltanski's photo installation of lost souls and Jeff Koons's stainless-steel bunny. This star-studded show may also be your only chance to see what the major dealers keep for themselves. "From Pop to Now: Selections from the Sonnabend Collection" at the Frances Young Tang Teaching Museum & Art Gallery at Skidmore College, Saratoga Springs, N.Y. (June 22—Sept. 29).