This chapter is adapted from the Leonard Diller Lecture that I was asked to give at the annual meeting of the Rehabilitation Psychology Division of the American Psychological Association in April 2008. In that talk, I drew on my long history of research collaboration with pioneers of cognitive rehabilitation research at New York University (NYU) Medical Center, particularly Leonard Diller, Yehuda Ben Yishay, and Joe Weinberg. In this critique of our current knowledge base with respect to cognitive rehabilitation, I begin with a brief look at the earliest studies conducted at NYU and then proceed to a summing up of where we are now, with an attempt to summarize what has been learned and what still needs to be accomplished.