This paper is basically the text of the Ken Lecture for 1991 (the 1990 Ken Award). The paper begins with some remarks about Dr. Kern. By, way of introduction to heat transfer enhancement, historical notes and the evolution of literature in this area are presented. Comments are made about the increasing practical applications of enhancement technology. Developments in single-phase convection are presented, with particular emphasis on offset strip fins and twisted-tape inserts. Pool boiling and flow boiling (particularly microfin tubes) are then considered in some detail It is concluded that enhancement represents a powerful technology to improve heat exchanger performance.