Keller's Personalized System of Instruction dominated the literature in the teaching of psychology and behavior analysis in the 1970s and 1980s. After this brief flourish of interest, PSI research trickled off to a nearly imperceptible stream in the 1990s. However, with the increasing availability and ease of use of computers and the internet, along with demand created by an ever-growing need for distance education, PSI is beginning to curry favor among a new generation of faculty members. The question of whether this resurgence of interest will be permanent revival or fleeting fancy will depend on how researchers deal with the PSI paradoxes that stymied the researchers of the 1970s and 1980s. This article will review the current state of PSI research including the riddles left to be untangled, illustrate how computers have affected the PSI movement, and discuss the role PSI might play in distance education. Keywords: Personalized System of Instruction, PSI, Mastery-Based Learning, Distance Education, Fred Keller, Keller Method, Keller Plan, Mastery.