Intravenous inotuzumab ozogamicin (Besponsa®; Pfizer) is an anti-CD22 monoclonal antibody-calicheamicin conjugate that binds to CD22-expressing tumour cells. Upon binding, the complex is internalised and the cytotoxic calicheamicin derivative is released inside the cell, inducing double-strand DNA breakage and subsequent cell death. In June 2017, the EMA granted inotuzumab ozogamicin approval as monotherapy for the treatment of adults with relapsed or refractory CD22-positive B-cell precursor acute lymphoblastic leukaemia (ALL). The use of inotuzumab ozogamicin in adult patients with Philadelphia chromosome-positive, relapsed or refractory CD22-positive B-cell precursor ALL is restricted to those who have failed treatment with at least one tyrosine kinase inhibitor. Inotuzumab ozogamicin was granted priority review for the treatment of relapsed or refractory B-cell precursor ALL by the US FDA in February 2017. In the USA, a phase III trial evaluating inotuzumab ozogamicin in combination with frontline chemotherapy in adults with newly diagnosed B-cell ALL has recently been initiated and inotuzumab ozogamicin is under phase II evaluation in childhood CD22-positive B-cell ALL. Inotuzumab ozogamicin combination therapies are also being evaluated in the phase I/II or II setting in ALL and chronic myeloid leukaemia and in the phase I setting in Burkitt's lymphoma. This article summarises the milestones in the development of inotuzumab ozogamicin leading to this first approval for ALL.