The papers in this special issue describe research undertaken by the MacArthur Foundation-funded Research Network on Youth Mental Health. The project is designed to understand the challenges of implementing evidence-based treatments in community-based mental health practices. This Introduction and the following articles describe the impetus and conceptual framework underlying one cluster of the Network's activity-i.e., the Clinic Systems Project (CSP). The CSP studies examined the organizational and service system environments in a large national sample of community mental health and affiliated organizations that provide services to children. The main goal is to identify leverage points for, and barriers to, the adoption and implementation of evidence-based practices for children.