Medical guidelines are constructed with the aim of assisting clinicians in making decisions that are informed by the best available medical evidence. In order to achieve this aim, they must be disseminated in a form that makes them easy for clinicians to use and easy for domain experts to critique. Furthermore, the language in which they are expressed must facilitate their transfer between institutions and their adaptation to local conditions. This paper describes PROforma, a knowledge representation language that attempts to meet these desiderata. PROforma is formal knowledge representation language designed to capture the content and structure of a clinical guideline in a form that can be interpreted by a computer. The language embodies many contemporary themes in machine interpretable guideline representation schemas whilst retaining some distinctive features. This paper describes the key features of the PROforma language in the context of recent trends and developments in the field of electronic guideline representation formats. We describe our experiences in applying PROforma to a range of clinical decision making and workflow problems, and the benefits and limitations of the current language specification are discussed. Finally, we outline plans for the further refinement of PROforma.