A sentinel event is a significant negative outcome that: 1) signals underlying weaknesses in the system or process; 2) is likely the result of compound errors; and 3) may provide, if properly analyzed and addressed, important keys to strengthening the system and preventing future adverse events or outcomes. NIJ's Sentinel Events Initiative borrows extensively from medicine and aviation (and other industries) where a blame-placing, backward-looking review process is yielding to a more forward-thinking, non-blaming, problem-solving approach. As in these other fields, significant advances in criminal justice processes may be achieved through the combined efforts of researchers, system analysts and the broad span of practitioners whose work is inextricably linked to the occurrence of error and the eventual strengthening of the system and the prevention of future system errors.