Health interventions are purposeful activities intended to improve health. They may involve treatment or care of the ill, or health promotion to prevent disease and illness. Complex health interventions have multiple components interacting with each other and with the context of delivery. Evaluation is important to ensure that complex health interventions are effective in achieving their intended outcomes, represent good value for money and cause minimal harm. Evaluation is also important to detect if interventions reduce or increase health inequalities. Intervention effects are not always obvious. They can easily be confused with other changes that occur. Hence, there is a need for evaluation to use rigorous methods to distinguish the ‘signal’ of intervention effects from the ‘noise’ of other effects in the absence of intervention. Evaluation should provide evidence to inform policy. If not based on evidence, there is a risk that policies may not achieve their intended effects or may create harms.