Uncertainty is ever-present when doctors make decisions. For general practitioners (GPs) the task of diagnosing and prognosing is particularly difficult. They are faced with a broad range of unselected patients and their problems. For this particular task they have developed strategies that are adapted to the ecology of their practice. Some of these rules and heuristics are intended to reduce the likelihood of harm to the patient, e.g. missing a serious disease. Others help doctors and their patients to deal with uncertainty on a subjective level. Cognitive processes of practicing doctors are still far from being elucidated. However, reflecting on their work will help professionals improve their development and the quality of their work.