Collecting and analysing reports of suspected adverse reactions to veterinary medicinal products is an important part of the regulatory process. The information obtained contributes to the body of knowledge of the adverse effects of a veterinary medicine and its constituents, further defines its safety profile and, where necessary, permits alterations to the terms of the marketing authorisation through restrictions or modifications to the terms of the authorisation, including changes to the product label and product literature. In some cases, authorisations may be suspended or revoked if the adverse reactions are deemed to be sufficiently serious or if mitigating measures are impractical. This chapter examines the European Union requirements for veterinary pharmacovigilance and the guidelines that are applicable. It is important to stress that not all adverse drug reactions have a toxicological basis, for example lack of expected efficacy and reactions to biological products. However, others do have a toxicological origin, and some examples of such adverse reactions to veterinary medicinal products in animals and in humans are reviewed.