This document discusses different types of device models. It covers classifications based on the rate of change of voltage/current, frequency, attributes of mathematical functions used, parameters/constants, and applications. Analytical models can use algebraic or non-algebraic functions, and be implicit or explicit. Models are also classified as empirical, semi-empirical/physical, or physical based on parameter meanings. Circuit models include table lookup, black box, behavioral, sub-circuit/macro, and compact models. Digital/analog models differ in goals and model equation complexity. Inverse modeling extracts device structure from measurement data.