An important part of drug development relies on the analysis of the relationships between drug doses and therapeutic and/or side effects. This analysis implies an in-depth understanding of the pharmacokinetics (PK) and pharmacodynamics (PD) of the drug and of the relationship which links them (PK-PD relationship). The aim of this round table was to define the place of the study of PK-PD relationships in drug development. After reviewing the definitions of PK models, PD models, and of integrated PK-PD models, the article highlights the importance of studying the PK-PD relationship during the successive phases of drug development (pre-clinical, phase I/II, phase III) and in specific populations (children, elderly people). A number of examples taken from pharmaceutical development or international literature are given. They show the methodology used and the type of information which can be drawn at each step of drug development. The article also presents the difficulties which prevent a more systematic application of this kind of approach during drug development. Scientific limits, problems in relation with the misunderstanding of the approach both in academic institutions and in pharmaceutical companies, and difficulties related to the lack of specific guidelines are discussed. The conclusion emphasizes the importance of using PK-PD modeling all along drug development and presents a number of actions which could further broaden its use.