There is rapidly growing concern about the problem of contamination of groundwater and aquifers by pollution, whether originating from landfill leachate, toxic wastes or petroleum spills. There is thus a clear need to understand the way in which chemicals interact with the soil, move with the flow of water and air and through the subsurface environment, and are transformed or biodegraded by biotic and abiotic processes. The fourteen papers in this volume represent the key presentations of a highly successful symposium at which 175 participants from fifteen countries discussed the issues. Topics include sorption, dissolution, volatilization, diffusion and multi-phase flow. Experimental investigations were stressed, but presentations were also included that were oriented toward simulation of transport if they emphasized fundamentals of processes affecting the transport and fate of contaminants.