  This tutorial article introduces the physics of spin transfer torques in
magnetic devices. We provide an elementary discussion of the mechanism of spin
transfer torque, and review the theoretical and experimental progress in this
field. Our intention is to be accessible to beginning graduate students. This
is the introductory paper for a cluster of "Current Perspectives" articles on
spin transfer torques published in volume 320 of the Journal of Magnetism and
Magnetic Materials. This article is meant to set the stage for the others which
follow it in this cluster; they focus in more depth on particularly interesting
aspects of spin-torque physics and highlight unanswered questions that might be
productive topics for future research.
