Adverse drug reaction (ADR) monitoring and its clinical application are also known as pharmacovigilance. WHO defines pharmacovigilance as “the science and activities relating to the detection, assessment, understanding and prevention of ADRs or any other, medicine-related problem”. Pharmacovigilance plays a major role in pharmacotherapy decision-making, be it individual, regional, national or international. Health professionals are in the best position to report on suspected ADRs observed in their everyday patient care. All healthcare providers should report ADRs as part of their professional responsibility, even if they are doubtful about the precise relationship with the given medication. Premarketing clinical trials and postmarketing methods are main methods used for collection of information on adverse reactions to drugs have been reported in literature. Three figures are also presented in this chapter.