The author defines pharmacovigilance as the process of identification, reporting, recording and systematic evaluation of the Adverse Drug Reactions (ADRs). After emphasizing the importance of the ADRs as a morbid entity, a few basic definitions are proposed. The main criteria of classification of ADRs are presented, and the most important points regarding the evaluation of causality between the administration of a medicine and an adverse reaction are discussed. The author points out the importance of the ADRs Spontaneous Reports as a tool of pharmacovigilance, a modern and efficient rational way of evaluating the risk-benefit ratio of the commercialized and developing drugs.