It is a truism of science that the more fundamental the subject, the more universally applicable it is. Nevertheless, it is important to strike a level of “fundamentalness� appropriate to the task in hand. For -depth study of the mechanics of motor cars would tell one example, an in nothing about the dynamics of traffic. Traffic exists on a different “level� - it is dependent upon the existence of motor vehicles but the physics and mathematics of traffic can be adequately addressed by considering motor vehicles as mobile “blobs�,with no consideration of how they become mobile. To start a discourse on traffic with a consideration of the mechanics of motor vehicles would thus be inappropropriate. In writing this volume, I have wrestled with the question of the appropriate level at which to address the physics underlying many of the techniques used in protein isolation. I have tried to strike a level as would be used by a mechanic (with perhaps a slight leaning towards an engineer) - i.e. a practical level, offering appropriate insight but with minimal mathematics. Some people involved in biochemical research have a minimal grounding in chemistry and physics and so I have tried to keep it as simple as possible.