Very sad news. Anyone who has studied art in the past 40+ years has been touched by Mr. Steinberg.
As reported on ArtInfo.com :
NEW YORK— Leo Steinberg, the Russian-born art historian and Renaissance scholar whose work had a heroic impact on the understanding and reception of postwar American art, particularly Pop and postmodern art, passed away in New York on Sunday at the age of 90. His death was reported in an email circulated by Utrecht-based art historian Gary Schwartz, and confirmed by University of Pennsylvania art history department chair Professor Holly Pittman.
A writer and thinker of immense erudition, Steinberg is considered alongside art critics Clement Greenberg and Harold Rosenberg as one of the most influential voices in shaping the way art was discussed and looked at in 20th century America. Writing and teaching in an era when the reception of new art was still largely codified by university departments, Steinberg electrified the art world in 1972 with his classic book "Other Criteria," which introduced the notion of the "flatbed picture plane" — a potent entry point for grasping the dimension-annihilating flatness of work by Jackson Pollock and Jasper Johns — through essays on those two artists as well as Picasso, Philip Guston, Robert Rauschenberg, and Willem de Kooning.
An art historian whose career began with a lively lecture series at the 92nd Street Y titled "An Introduction to Art and Practical Esthetics," Steinberg taught over the years at universities including Penn, Columbia, and Harvard. In addition to his work on postwar art, he is highly regarded for such seminal works of classical art history as "The Sexuality of Christ in Renaissance Art and in Modern Oblivion," and his critical Picasso touchstone "The Philosophical Brothel" about "Les Demoiselles d' Avignon." Unusual among art historians of his traditional background, Steinberg favored writing in an accessibly jargon-free first-person style replete with anecdotes and a humane subjectivity.