This is a novel approach to teaching dynamic aspects of the operation of semiconductor and opto-electronic devices. The traditional approach emphasizes an understanding of the steady equilibrium operation. However, dynamic aspects often determine the steady state conditions and the dynamical operation is of increasing importance as modern methods of communicating data and information require electronic devices that switch electrical or optical signals at ever faster rates. The opening chapter considers a number of simple problems, several drawn from daily experience, where the rates of movement can be used to determine equilibrium states. The remainder of the book concentrates on specific problems in semiconductor physics: the rates at which transistors and diodes can switch, and the rates at which electrons and holes can interact with photons, and photons with photons.