Chronic lymphocytic leukemia is a mature B-cell neoplasm characterized by the proliferation of small monomorphic cells, representing the most frequent type of leukemia in Western countries. Small lymphocytic lymphoma is considered the same entity, but in the presence of <5 × 109/L circulating neoplastic B cells and a solid tissue involvement. Chronic lymphocytic leukemia/small lymphocytic lymphoma aberrantly expresses the T-cell marker CD5, and most patients are adults aged 50–70 years, with a slight female predominance. The oral cavity is more frequently affected as a manifestation of a disseminated disease, exhibiting an asymptomatic expansive swelling. The tonsils, palate, and gingival tissue are the most affected locations. Richter transformation represents the occurrence of high-grade transformation in a previous chronic lymphocytic leukemia/small lymphocytic lymphoma, more often a diffuse large B-cell lymphoma. In the oral cavity, some cases of Richter transformation have also been reported, usually describing painful swellings that may be ulcerated. Morphologically, chronic lymphocytic leukemia/small lymphocytic lymphoma shows a diffuse infiltration of small and monomorphous neoplastic cells, with hyperchromatic nucleus and scant cytoplasm, expressing CD20, CD5 and CD23 proteins. This neoplasm remains an incurable disease, despite improvements in the therapeutic management of patients.