These proceedings of a workshop held at Isle-aux-Coudres, Quebec, Canada in August 1993, provide a comprehensive, up-to- date assessment of a fundamental subject in modern stellar astrophysics. Hot, luminous stars play a crucial role in the rapid recycling of matter in dense regions of the Universe. The most luminous objects generate the strongest outflows of material, which in turn have a profound impact on both the interstellar medium and the underlying stars themselves. However, recent observational and theoretical advances have shown that these outflows are highly unstable and ...