The monitoring of drug safety is essential so as to be sure that the drugs that are on the market are safe, effective and of good quality. The activity that monitors this drug safety is called pharmacovigilance and consists in detecting, assessing understanding and preventing adverse drug reactions or any other drug-related problem. This thesis aims to describe the process of drug safety throughout the drug's life-cycle, process that goes hand-in-hand with every stage of the development of the drug but continues also in the postmarketing phase once the drug is given to real-life patients. After going through the history of drug safety and the burden of adverse drug reactions, we attempt to put forward the usefulness of clinical trials in acquiring a first safety profile of a new drug in a carefully selected population but also the limitations of these clinical trials in detecting rare, unknown adverse drug reactions in real-life situations. The different means by which the gaps in drug safety can be further filled in are explained, from spontaneous adverse drug reporting systems to pharmacoepidemiologic studies, means that allow collecting data on special populations such as elderly patients, children and pregnant women or in special situations such as drug misuse, drug-drug interactions and particular pharmacogenetic profiles. Finally, we review how the monitoring of drug safety can be further improved, using new technologies and better communication.