Abstract
Non-monotonic reasoning is reasoning that will deny conclusions in the light of new evidence. This kind of reasoning is important for many common sense phenomena. In particular it is useful in reasoning about change. Circumscription is a common methodology for capturing non-monotonic inferences. I characterize its expressive power, and show how it can be extended, to be more tolerant of new defaults, and so that it can capture defaults that current versions cannot express. Examples from reasoning about action are used as the primary motivating tool.