On October 1995, the medical faculty of the University of Geneva has started a new second and third years preclinical curriculum. It consists of 16 integrated "Problem-Based Learning (PBL) Units". The first "Introduction" Unitof2 weeks is aimed at introducing students with the PBL technique. Eleven PBL Units each of one month duration, comprise of 7 to 8 problems with themes related to important body functions or organ. Finally 4 Synthesis Units, each of 2 weeks duration, and following 2 to 3 PBL Units, comprise 4 problems aimed at integrating concepts common to preceding Units. The elaboration of such a new PBL curriculum requires to address two main issues, namely 1) how to select the important basic medical concepts and relevant problems leading to the learning objectives of each learning Unit and 2) how to verify the quality and relevance of the designed problems. This paper is aimed mainly at describing the steps followed in Geneva to solve these issues. First, it will present the process of selecting the basic science concepts, designing the problems and establishing their sequence. Second, it will describe the process of verifying whether problems fit to the identified concepts, lead to learning objectives and reach the appropriate level of learning for the students.