BACKGROUND: In most Western countries the responsibilities of physicians include assessing work ability and issuing certificates for sickness absence and disability pension. These tasks often have a substantial impact on the lives of patients and constitute a financial burden on employers, insurance companies, and communities. AIM: The aim was to review scientific studies on sickness certification practices of physicians published in English, Danish, Norwegian, or Swedish. METHOD: Analyses were carried out of studies searched for through literature databases, reference lists, and personal contacts. RESULTS: Twenty-six publications fulfilled the inclusion criteria. Most of these were published in the last decade. The studies focused on physicians, and occasionally on physicians and patients, but never on interaction between them. Data had generally been collected using questionnaires, some including case vignettes. There was a large variation in how long different physicians sickness-certified similar patients. Three comprehensive categories of studies were identified dealing with the following: (1). how physicians certify sickness; (2). factors that might affect the certification process; (i.e. elements related to the patient, to the physician, or to restrictions in insurance legislation); and (3). studies concerning attitudes. No studies were found that took into account the work conditions of the hospital or health-care organization in which the physician works. CONCLUSIONS: The research problems were seldom medical in nature but were instead carried out within the realm of behavioural science and should preferably be conducted using theories from behavioural, social, and public health scientific theories. Furthermore, factors such as gender, ethnicity, and power should be taken into consideration in studies on this complex phenomenon.