This chapter includes general aspects regarding the definitions and mechanisms of occurrences of adverse drug reactions (ADRs). They can be realized through non-immunological (type A reaction) or immunological (type B reaction) pathways and can be dose-dependent or independent. A new type of ADR is encountered in oncology departments in patients taking monoclonal antibodies. It is known as drug-induced apoptosis and is presented in this chapter. The mechanisms and classification of the severity of these reactions, as well as the particularities of acute, chronic and chronic-delayed ADRs, are also explored. The severity can be patient- or drug-related. In the chapters that follow, specific system- and organ-related ADRs are presented.