This text describes analytical compact transistor models that can be used in circuit simulation programs like SPICE. It provides the reader with a thorough knowledge of many aspects of compact models. The book starts with the necessary device physics: Boltzmann transport equation; continuity equation; Poisson equation; and physical modelling of mobility, recombination, bandgap narrowing, avalanche multiplication and noise. Then, a systematic treatment of the analytical formulas that describe the device behaviour in DC, AC and transient situations is given for both bipolar and MOST devices. The book contains complete sets of model equations for various models, including some new ones, and special attention is paid to the numerical problems of analytical continuity. Separate chapters are devoted to parameter determination, the parameter temperature dependence, as well as their relation to process variables, the statistical correlations between parameters, the scaling rules for submicron devices, the side-wall effects and the parasitics.