Chronic lymphocytic leukemia (CLL) is a slow-growing cancer of the blood and bone marrow characterized by the excessive production and accumulation of abnormal B lymphocytes, which interfere with normal immune function. It most commonly affects older adults and may initially cause no symptoms. As the disease progresses, patients may experience fatigue, swollen lymph nodes, frequent infections, fever, night sweats, unexplained weight loss, easy bruising or bleeding, and an enlarged spleen. Diagnosis is based on blood tests such as a complete blood count, peripheral blood smear, flow cytometry, and sometimes bone marrow examination with genetic testing. Treatment depends on the stage and severity of the disease and may include active surveillance (watchful waiting), targeted therapy, immunotherapy, chemotherapy, or stem cell transplantation in selected cases. The prognosis is generally favorable, with many patients living for years due to the slow progression of the disease and advances in modern treatment.