Book excerpt: Chronic lymphocytic leukemia (CLL) is a slow-growing type of blood cancer and the most common form of leukemia in adults. CLL results in large numbers of abnormal B lymphocytes in the bone marrow and prevents the production of healthy blood cells. It is more commonly diagnosed in older people: almost 80% of cases are in people over 60 years old. Changes in chromosomes or genes in some patients affect how the disease develops and what treatment is prescribed. CLL cannot be completely cured but many people will have a normal lifespan and a good quality of life.