The following papers were presented as contributed talks at the Chapman Conference on Plasma Waves and Instabilities in Magnetospheres and at Comets held in Sendai/Mt. Zao, Japan, October 12–16, 1987. The tutorials and invited talks given at the conference will appear in an AGU monograph at about the same time as the issue of theJournal of Geophysical Research.The purposes of the conference were (1) to discuss recent discoveries of plasma waves and instabilities at comets and nonlinear processes and turbulence in plasmas, particularly computer simulation results, and (2) to compare and contrast instabilities in the magnetospheres of the Earth, Jupiter, Saturn, and Uranu