The collective model of nuclei is an attempt to explain certain striking properties of nuclear spectra and reactions in terms of only a few degrees of freedom, chosen to represent gross properties of the nucleus such as its shape and orientation. This possibility is suggested by analogy with macroscopic systems, such as rigid bodies or drops of liquid, whose behavior in most practical conditions can be satisfactorily described and predicted in terms of similar average properties such as shape, orientation, flow patterns, temperature, etc. In these macroscopic systems, it is seldom necessary to describe in detail the motion of the many constituent atoms and electrons, even though these ultimately make up the motion and so determine it. Instead, the macroscopic variables seem to be enough not only to describe but also to predict their own motion. The laws governing macroscopic motion were discovered heuristically, and in general they take the form of equations involving a few, empirically determined coefficients.