B-cell Chronic Lymphocytic Leukemia (CLL) is a disease which has a variable course, and survival rates range from months to decades. It is evident that clinical heterogeneity represents biologic variety with two main subtypes in terms of cellular multiplication, clinical aggressiveness and predictability. As CLL progresses, irregular hematopoiesis leads to pancytopenia and decreased immunoglobulin creation, followed by nonspecific symptoms such as fatigue or malaise. A cure is tough to find in normal cases and postponed treatment (until symptoms develop) is aimed at lengthening life and eliminating symptoms. Experts are playing a crucial role in studying CLL's main source and the role of genetics in the development of this disorder. Research programs are devoted towards comprehending the essential mechanisms underlying CLL with the hope of enhancing treatment options. The book discusses CLL biology and microenvironment, and CLL animal models.