This specialized volume of Advances in Cancer Research is dedicated to the complex and largely unexplored field of intratumor diversity, with respect to differences in both genotype and phenotype among cell populations within tumors. - Unique subject matter: there are relatively few previous publications covering this complex topic, even though it has been a matter of much speculation ever since cancers were first studied under the microscope more than a century ago. This volume is an attempt to update the reader on novel findings as well as giving a historical perspective to the field. It thereby offers an excellent starting point for young scientists entering this field of research - Contributors: many of the contributors are renowned cancer researchers with long-term experience. To bring in new ideas to the field, these experienced authors have been blended with young bright minds who might provide novel starting points for solving old problems - Technical aspects: some chapters will be focused on specific techniques to address intratumor diversity and will therefore be helpful for colleagues wanting to broaden their technical expertise