Health interventions vary substantially in the degree of effort required to implement them. To some extent this is apparent in their financial cost, but the nature and availability of non-financial resources is often of similar importance. In particular, human resource requirements are frequently a major constraint. We propose a conceptual framework for the analysis of interventions according to their degree of technical complexity; this complements the notion of institutional capacity in considering the feasibility of implementing an intervention. Interventions are categorized into four dimensions: characteristics of the basic intervention; characteristics of delivery; requirements on government capacity; and usage characteristics. The analysis of intervention complexity should lead to a better understanding of supply- and demand-side constraints to scaling up, indicate priorities for further research and development, and can point to potential areas for improvement of specific aspects of each intervention to close the gap between the complexity of an intervention and the capacity to implement it. The framework is illustrated using the examples of scaling up condom social marketing programmes, and the DOTS strategy for tuberculosis control in highly resource-constrained countries. The framework could be used as a tool for policy-makers, planners and programme managers when considering the expansion of existing projects or the introduction of new interventions. Intervention complexity thus complements the considerations of burden of disease, cost-effectiveness, affordability and political feasibility in health policy decision-making. Reducing the technical complexity of interventions will be crucial to meeting the health-related Millennium Development Goals.