This volume contains the notes of lectures given at the school on "Nonlinear Dy­ namics in Solids" held at the Physikzentrum Bad Honnef, 2-6 October 1989 under the patronage of the Deutsche Physikalische Gesellschaft. Nonlinear dynamics has become a highly active research area, owing to many interesting developments during the last three decades in the theoretical analysis of dynamical processes in both Hamiltonian and dissipative systems. Research has been focused on a variety of problems, such as the characteristics of regular and chaotic motion in Hamiltonian dynamics, the problem of quantum chaos, the formation and properties of solitary spatio-temporal structures, the occurrence of strange attractors in dissipative systems, and the bifurcation scenarios leading to complex time behaviour. Until recently, predictions of the theory have been tested predominantly on insta­ bilities in hydrodynamic systems, where many interesting experiments have provided valuable input and have led to a fruitful interaction between experiment and theory. Fluid systems are certainly good candidates for performing clean experiments free from disturbing influences: with fluids, compared to solids, it is simpler to prepare good samples, the relevant length and time scales are in easily accessible ranges, and it is possible to do measurements "inside" the fluid, because it can be filled in after the construction of the apparatus. Further, the theory describing the macroscopic dynamics of fluids is well established and contains only very few parameters, all of which have well-known values.