Agonistic antibodies mimic the activities of native ligands by initiating signalling by the receptors they target. As stimulators of immune-cell surface receptors, they offer the prospect of resetting immune responses in autoimmune conditions or initiating new or stronger reactions in contexts such as cancer immunotherapy. Despite these theoretical advantages and preclinical promise, the use of agonistic antibodies has so far delivered only limited benefit to patients. Recently, however, considerable progress has been made in understanding their mechanisms of action, which holds the key to engineering new antibodies more likely to realize the clinical potential of this class of biologics. Agonistic antibodies that target tumour necrosis factor receptor (TNFR) superfamily proteins in the context of anti-cancer therapy have been a major focus of these studies, but substantial progress has also been made in understanding how antibodies and antibody-like molecules trigger signalling by immune receptors and cytokine receptors, creating opportunities to treat autoimmune disorders. Here, we summarize recent progress in understanding how antibodies initiate receptor signalling. We also review the clinical landscape for agonistic antibody-based immunotherapy and discuss how newly gained mechanistic insights should broaden the clinical scope and improve the safety and efficacy of the approach.