Once the parameters to design and assess a Mental Health System have been put forward it's necessary to operationalize them into quality indicators and outcome measures. These indicators allow us to compare processes and results with other systems and services, they let us do the economic assessment and the allotment of resources, they give way to treatment guidelines and external evaluation, generating a new type of organizations evidence-based. This paper reviews the conceptual aspects of performance indicators and its use in some developed countries. It also discusses their usefulness, at the input, the process and the outcome level, to treat patients and to organize Mental Health Services and Systems. Despite therapists not benefiting directly from the use of this type of indicators, its importance should not be disregarded in order to organize a Mental Health System for large populations.