Cell Biology plays a relevant role in understanding the effects of xenobiotics on organisms and ecosystems because the cell is the site of xenobiotic accumulation, metabolism and reaction, and offers tools and new approaches for the biological assessment of pollution, such as the use of cultured cells in toxicity testing and cellular biomarkers in environmental monitoring. This book comprises reviews of the existing knowledge and new ideas put forward during a course held in the University of the Basque Country in June 1994. It has been designed to provide a rational assessment of current potential methods and concepts to solve problems as well as directions for future research.