This text is aimed at a student readership but will also be useful to life science researchers faced with the task of isolating a protein. The logic of the overall approach to protein isolation is explained and the physical principles of each separation method are made clear by the use of simple models and analogies drawn from everyday experiences. The author's aim has been to deepen the readers' insight into protein isolation methods, so that they may tackle new problems and perhaps devise new approaches to old problems. many of the methods described are drawn from the author's own research and are thus uniquely described here; examples are three-phase partitioning, non-linear electrophoresis and a simple approach to buffer making. In this second edition, the treatment of the basic physical principles has been expanded and clarified, the importance of ionic strength in measuring enzyme pH optima is emphasised and a computer program for the calculation of buffers of defined ionic strength is provided. The section on three-phase partitioning has been expanded to include the latest research findings on the use of t-butanol to inhibit enzymes and minimise homogenisation artefacts, the treatment of HPLC has been expanded and the most common practical methods are explained in detail in a new chapter. Additional study questions are provided, as are the answers to all study questions.