As the advanced practice nursing initiative in Canada gains momentum, effort is being directed towards clarifying and defining advanced practice roles. A qualitative study was undertaken to increase understanding of the clinical nurse specialist role of advanced practice. Sixteen nurses who worked in advanced practice roles, organizing and providing healthcare for children with complex health needs and their families across the continuum of care, participated in in-depth conversations about the nature of their practice, the knowledge that informs it and the factors that influence it. Findings suggest that clinical nurse specialists have a unique role in the organization and delivery of healthcare for specialized populations with complex health needs in their dual focus on the system level of healthcare and on population health needs. Initiatives directed to children and families within the study participants' specialties included program development, consultation and educational outreach and the development of clinical guidelines and policies. Although the nurses described their practice as focusing both on individual children and families and on the population of children and families within their specialty, it is at the population level that they see their greatest potential for contributing to the delivery of high-quality, cost-effective healthcare.