This review is a survey of prospective studies on the clinical performance of posterior resin composites published between 1996 and 2002. Material, patient- and operator-specific data, observation periods, isolation methods of the operative field, and failure rates are detailed in tables. The data were evaluated statistically in order to assess the role of materials (filler size, bonding system, base materials [e.g. glass ionomer cements], and lining materials), study design, and personnel on failure rates. The primary reasons for composite failure were secondary caries, restoration fracture, and marginal defects. The influence of different commercial material brands on failure rates was not evaluated due to the great variety of test substances and the lack of material-specific documentation. Effects of the isolation method of the operative field (rubber dam or cotton rolls) and the professional status of operators (university or general dentist) on composite failure rates were not found to be significant. Observation periods varied from 1 to 17 years, and failure rates ranged between 0% and 45%. A linear correlation between failure rate and observation period was found (P<0.0001). Thirteen of 24 studies were terminated after 3 years, while seven studies continued for more than 10 years, indicating that favourable results for composite materials are frequently based on short-term results, despite higher dropout rates in longer studies. To determine accurately the risk for patients, long-term, randomised, controlled clinical trials of treatment outcomes with composites used in posterior teeth are clearly needed.