Biological therapies have improved the outcomes of several major inflammatory, autoimmune and also neoplastic disorders. Those directed towards cytokines or other soluble mediators, cell-surface molecules or receptors or various components of intracellular signalling pathways may be associated with the occurrence of infections whose diversity depends on the particular immune target. In this context and following a keynote lecture given by one of us at the European League Against Rheumatism meeting on June 2018, a multidisciplinary group of experts deeply involved in the use of targeted and biological therapies in rheumatoid and psoriatic arthritis decided to summarise their recent vision of the immunological basis and epidemiology of infections occurring during targeted and biological therapies, and provide useful indications for their management and prevention.