Introduction to the development of complex interventions.

Healthcare interventions are complex, but have the potential to deliver more efficient, cost-effective care and improved health outcomes. Careful attention must be paid to their early planning and development to minimise research waste or interventions that fail to deliver what they set out to achieve. The Medical Research Council provides guidance to help intervention developers, encouraging an explicit and iterative approach. This article describes the Medical Research Council's guidance and introduces two frequently used tools that further support the process of intervention design.