Children's mental health needs continue to be largely unmet, even when community services are provided. On-site mental health services in schools address unmet needs by improving access to, and relevance of services. As schools have increasingly been mandated to serve the needs of all children (including those who are emotionally disturbed) general health and mental health services have been increasingly placed in them. Although the provision of basic health care in schools began in the early part of the century, the concept of providing comprehensive services, in which mental health services are integrated into primary medical care, has only been implemented recently. We review the background of school-based mental health services, compare various models of service delivery, and highlight issues important to the future development and advancement of these services.