Current leading-edge CMOS transistors are about as small as they will get. We now have a simple, clear, very physical understanding of how these devices function, but it has not yet entered our textbooks. Besides, CMOS logic transistors, power transistors are increasingly important as are III-V heterostructure transistors for high-frequency communication. Transistor reliability is also important but rarely treated in introductory textbooks. As we begin a new era, in which making transistors smaller will no longer be a major driving force for progress, it is time to look back at what we have learned in transistor research. Today we see a need to convey as simply and clearly as possible the essential physics of the device that makes modern electronics possible. That is the goal of these lectures. This volume rearranges the familiar topics and distills the most essential among them, while adding most recent approaches which have become crucial to the discussion. To follow the lectures, readers need only a basic understanding of semiconductor physics. Familiarity with transistors and electronic circuits is helpful, but not assumed.