|Abstract:||Security requirements change, but the typical way of improving system security by patches is ad hoc and has not produced good results. Security improvements should be systematic, just as new features can be added to software systematically. It would be easier to improve the security of a system if we had a catalog of security-oriented program transformations that could be used to plan changes, to divide the work to make changes, and as a target of automation. This paper describes a catalog of security-oriented program transformations that were derived from security patterns. It describes several ways of categorizing these transformations, our first attempts at validating the catalog, and how the catalog can be used to improve the security of systems.