This article is number 4 in a series of 21 articles on tools for evidence-informed health policymaking. Policymakers and those supporting them may find themselves in three situations that require them to characterise the costs and consequences of options to address a problem. These are:

1. A decision has already been taken and their role is to maximise the benefits of the option, minimise its harms, optimise the impacts achieved for the money spent, and (if there is substantial uncertainty about the likely costs and consequences of the option) to design a monitoring and evaluation plan

2. A policymaking process is already underway and their role is to assess the options being presented to them, or

3. A policymaking process has not yet begun and their role is to identify options, characterise their costs and consequences, and look for ‘windows of opportunity’ to act. Research evidence, particularly evidence about benefits, harms, and costs, can help to inform whether an option is considered viable.