Treatment of type 1 diabetes mellitus (DM1) has greatly improved but remains limited to combating the consequences of the disease. Target values for glucose regulation are achieved in only 20% of patients. Immunosuppression can slow disease progression, but does not cure DM1. Immunotherapy attempts to protect remaining insulin-producing β cells and β cell function. Promising results of immunotherapy in phase 2 studies in patients with DM1 could not be reproduced in phase 3 studies. These studies showed heterogeneity played a role in patient populations and between ethnic groups. In future studies better endpoints of efficacy, biomarkers of disease progression and response to therapy are essential. Vaccination with β-cell specific antigens to stimulate tolerance and vaccination combined with immunotherapy (biologicals) are options for future therapy. Discussion on the acceptability of the side effects of immunotherapy is desirable.