Chronic Lymphocytic Leukemia (CLL) is the most common leukemia in the western world, seen mostly in the elderly age-group and has a very variable clinical outcome. Traditionally considered an indolent, antigen inexperienced leukemia of slowly accumulating cells that do not die, researchers now acknowledge that CLL cells are highly proliferative, antigen experienced cells that have a high cell turnover and a subset show an aggressive clinical course. The onset of the disease is usually asymptomatic; only abnormalities in whole blood count such as leukocytosis with lymphocytosis are found. Nowadays, CLL is diagnosed more often at an early, asymptomatic stage due to more frequent routine blood tests. More advanced stages are characterized by lymphadenopathy, hepatomegaly/splenomegaly, recurrent infections, weakness, pallor and hemorrhagic diathesis, and general symptoms such as weight loss, fever and night sweats are observed. This book reviews the diagnosis, treatment options and prognosis of CLL.