In this review an update concerning the prognostic factors and scoring systems in peritonitis is given. The course of the disease is influenced by the physiological reserve of the patient, the acute severity and the type of the disease, the success of the operation and subsequent management and complications. The Acute Physiology and Chronic Health Evaluation Score (APACHE II) may be used with pre-operative data for the assessment of the severity of the disease or with postoperative data for monitoring. The Mannheim Peritonitis Index (MPI) and the Peritonitis Index Altona are based on pre- and intraoperative data and are also used for severity assessment. Treatment selection with respect to re-operation is supported by the Prognostic Peritonitis Model and the Abdominal Re-operation Predictive Index, covering data from the pre-, intra- and postoperative period. Adequate evaluation data are available for APACHE II and MPI, showing that these instruments are useful for risk assessment, definition of inclusion/exclusion criteria and analysis of comparability in clinical trials. Evaluation of treatment policies and quality assurance can best be performed with APACHE II, monitoring with APACHE II and Multiple Organ Failure scores. Support of individual patient care is still experimental.