For more than three decades the methods for assessing cell pro- liferation have been largely the preserve of experimental biologists, and in their hands such techniques have contributed greatly to our understanding of the dynamic organisation of normal and patho- logical tissues. In recent years, with the advent of novel method- ologies, there has been increased interest among both pathologists and clinicians, particularly oncologists and others interested in neoplasia, in assessing cell proliferation. This interest has been stimulated by the possibility that indices of cell proliferation may have direct clinical relevance, for example in being useful predictors of outcome in patients with certain forms of malignancy. In addi- tion, interest in assessing cell proliferation has been fuelled by the tremendous advances in our understanding of the mechanisms of cell proliferation and their deregulation in pathological processes. Consequently, the time is ripe for a monograph critically reviewing the available methods for assessing cell proliferation, their potential and their problems.We have been particularly concerned to present a balanced view of the advantages and disadvantages of different methods currently available for assessing cell proliferation. The assessment of cell proliferation often requires some familiarity with mathematical methods, but in this book we have attempted to keep detailed mathematical analyses to a minimum. We have asked exponents of each of the well-recognised methods to critically review the tech- niques and the ways in which they may be applied to clinical mate- rial.