Clinical guidelines are developed by multidisciplinary groups to advise practitioners, policy makers and commissioners on best practice in patient care and public health. Guidelines are perceived as highly influential, shaping both clinical practice and future research. Their prestige is predicated on their formulation from a scientific evidence base, yet we know little about how groups interpret and translate evidence into recommendations. How are decisions made when a guideline development group is confronted with an absence or paucity of evidence? What influences decision-making when there is more or less evidence?