Book excerpt: It is a truism of science that the more fundamental the subject, the more universally applicable it is. Neverthelens, it is important to strike a level of "fundamentalness" appropriate to the task in hand. For example, an in-depth study of the mechanics of motor cars would tell one 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.