This article is number 3 in a series of 21 articles on tools for evidence-informed health policymaking. Policymakers and those supporting them often find themselves in situations that spur them on to work out how best to define a problem. These situations may range from being asked an awkward or challenging question in the legislature, through to finding a problem highlighted on the front page of a newspaper. The motivations for policymakers wanting to define a problem are diverse. These may range from deciding whether to pay serious attention to a particular problem that others claim is important, through to wondering how to convince others to agree that a problem is important. Struggles over how to define a problem are a critically important part of the policymaking process. The outcome of these struggles will influence whether and, in part, how policymakers take action to address a problem. When defining a problem, those efforts that are informed by an appreciation of concurrent developments related to policy and programme options (e.g. the publication of a report demonstrating the effectiveness of a particular option) and of concurrent political events (e.g. the appointment of a new Minister of Health with a personal interest in a particular issue) are more likely to generate action.