The dialyser and its centrepiece, the membrane, play a central role in chronic renal replacement therapy. More than 1000 different types of dialysers are currently available on the market. How are these characterised and how do they differ? What are their special features and how do they contribute to acute and chronic effects in the patient? Is there a link between morbidity and mortality and the use of a particular type of dialyser? This book addresses these questions and attempts answers based on current scientific knowledge. In this context, dialyser development and the basics of filter performance and biocompatibility assessment are reviewed.