Chronic lymphocytic leukemia (CLL) is the most common leukemia in the Western world. CLL has a highly varied clinical course. While advances in CLL therapy are noted, many patients still succumb to this illness. Like most progress in medicine, solid advances in the diagnosis, prognosis and treatment of CLL are rooted in an in-depth understanding of the basic and translational biology of CLL. In this book, CLL experts have contributed state-of-the-art summaries of various important aspects of CLL biology and have discussed the translational implication of such findings. This book, which is directed at physicians and researchers alike, aims to educate broadly and deeply. Intentionally, the many aspects and nuances of CLL clinical care that can only really be appreciated through direct patient care are not covered here, but instead, the book presents basic aspects of CLL that underlie many of the contemporary decisions that are made in CLL research and clinical settings. We hope that this book will critically inform the community and stimulate interest in CLL, which will ultimately translate into better CLL research, prognostication and therapy, with the end goal of providing a better outlook for patients afflicted with this common leukemia.