Implementation fidelity refers to the degree to wich an intervention is delivered as intended. Implement evidence-based interventions with high fidelity is a key issue in the dissemination of programs to real life contexts, since the effects of the programs diminish when programs are implemented with poor fidelity to the protocol designed. This study defines the concept of fidelity and its component dimensions. In turn, we review the most relevant aspects related to implementation fidelity of school-based substance use prevention programs. Finally, it is discuss the importance of increasing our knowledge about the process of implementing the innovations of science in daily practice.