Density profiles of most globular clusters are well fitted by a King (1966) model. The evolution of a King model in the tidal gravitational field of the Galaxy is first discussed. If the concentration parameter c (= log(rt/rc)) is small enough, the evolution is nearly along the King model sequence and c becomes larger. When c becomes large enough (about 2.1), gravothermal instability sets in. The basic properties of gravothermal instability is next discussed. The stability criterion and its interpretation are given. Globular clusters consist of stars with disparate masses, so that finally the evolution of multi-component clusters is discussed. Acceleration of evolution in multi-component clusters and equipartition of the kinetic energies among components are discussed, and conclusions and future problems are given.