Consultation-liaison (C-L) psychiatry is hypothesized to be a model of interface between primary care and specialist mental health services with significant advantages over other models of organizing mental health care. However, there are significant complexities in the definition and evaluation of this model. As well as discussing the definition of C-L in primary care, this paper highlights the gap between models of traditional C-L that are popular in practice and the increasingly complex models (based on chronic disease management) evaluated in research studies. It is hypothesized that traditional C-L approaches and newer models use different mechanisms of change to achieve their goals. The former focus on the relationships between primary care and specialist professionals, while the latter highlight the importance of the development of effective systems of delivering care. Although the latter may be crucial in enhancing the "efficacy" and "effectiveness" of these models in terms of clinician behavior change and patient outcome, the former may be crucial in terms of "dissemination" and "implementation" of these models from research contexts to routine care settings.