This text covers the latest approaches to improving the cell host through improved understanding of the molecular biology, the development of novel vaccines, approaches to bio-reactor design and operation, monitoring techniques in process control and quality related topics. The work was carefully put together as one result of the Cell Culture Engineering IV Meeting held in San Diego, USA in 1994. It is designed for cell biologists, biochemists, molecular biologists, immunologists and other disciplines related to cell culture engineering, working in the academic environment, as well as in the biotechnology or pharmaceutical industry.