Inotuzumab ozogamicin is an antibody-drug conjugate comprised of a humanized anti-CD22 monoclonal antibody conjugated to calicheamicin, a cytotoxic antibiotic agent. Inotuzumab ozogamicin binds to CD22-expressing tumor cells, resulting in apoptotic cell death. Based on the results of the pivotal, phase III INO-VATE trial in acute lymphoblastic leukemia (ALL), approval of inotuzumab ozogamicin was recently granted for the treatment of patients with relapsed or refractory ALL, a group that otherwise has a poor prognosis with standard chemotherapy. Several ongoing clinical trials are now testing whether outcomes can be further improved by combining inotuzumab ozogamicin with low-dose chemotherapy or by including inotuzumab ozogamicin in the front-line setting. In this article we discuss the preclinical, clinical and safety data of inotuzumab ozogamicin.