Developers of complex systems have to address concerns such as security, availability of services, and timeliness that often are non-orthogonal to traditional design structures, that is, the concerns cross-cut traditional design units. In this paper we illustrate how an aspect-oriented approach to modeling allows developers to encapsulate such design concerns so that they can be woven into a design in a systematic and consistent manner. The paper focuses on the use of aspects for modeling and weaving in security concerns.