Heterogeneous catalysis has been essential to the development of efficient chemical processes for more than a century, and this field has been traditionally part of the solid state chemistry and surface science communities. The design of better catalysts has raised the following questions: "what is the structure of the active sites" and "how to control their nature" The necessary need to develop more sustainable chemical processes and the success of homogeneous catalysis relying on molecular organometallic chemistry has led the community of molecular chemists to investigate the preparation of single-site heterogeneous catalysts. The authors discuss the molecular design, the preparation, the characterisation and the catalytic applications of well-defined oxides and metal particles. The readers will acquire a molecular understanding of heterogeneous catalysis, which will help them develop a critical view and which will attract them to study this fascinating field.