Published online by Cambridge University Press: 14 August 2015
The basic ideas and some of the most important recent developments of the gravitational theories of spiral structure are described. A separation between linear and non linear effects is made. The linear self consistent problem consists of the problem of modes and of the initial value problem, which is discussed here in some detail. More emphasis is put on the non linear problem near resonances and in particular the inner Lindblad resonance. The linear density response to a slightly growing spiral potential (trailing or leading) near the inner Lindblad resonance is always trailing, while non linear effects form a density distribution with a roughly quadruple symmetry.