This review summarizes the events ruled by CD38 in shaping the bone marrow environment, recapitulating old and new aspects derived from the body of knowledge on the molecule. The disease models considered were myeloma and chronic lymphocytic leukemia (CLL). CD38 has been analyzed considering its twin function as receptor and enzyme, roles usually not considered in clinics, where it is used as a routine marker. Another aspect pertaining basic science concerns the role of the molecule as a member of an ectoenzyme network, potentially metabolizing soluble factors not yet analyzed (e.g., NAD+, ATP, NAM) or influencing hormone secretion (e.g., oxytocin). The last point is focused on the use of CD38 as a target of an antibody-mediated therapeutic approach in myeloma and CLL. A recent observation is that CD38 may run an escape circuit leading to the production of adenosine. The generation of local anergy may be blocked by using anti-CD38 antibodies. Consequently, not only might CD38 be a prime target for mAb-mediated therapy, but its functional block may contribute to general improvement in cancer immunotherapy and outcomes.