Chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/ SLL) is the most common adult leukemia in Western countries. The Surveillance, Epidemiology, and End Results (SEER) data have shown the incidence of CLL/SLL in the United States has been steadily rising on average by 0.2%. each year. The classic features of CLL include a circulating monoclonal B lymphocyte population with varying degrees of cytopenias and hepatosplenomegaly. Bone marrow (BM) biopsy is recommended for all patients who have cytopenias of unknown cause or prior to therapy initiation in order to determine the BM cellularity and degree of CLL infiltration. CLL and SLL are considered the same entity by the World Health Organization (WHO) based on the commonality of immunophenotypic and pathologic features. Patients with CLL/SLL can also undergo Richter’s transformation into more aggressive diffuse large B-cell lymphoma.