Many modification tasks on an existing software system result in changes to code that crosscuts the system's structure. Making these changes is difficult because a developer must understand large parts of the system and must reason about how the modification will interact with the existing behaviour. Most of the time, developers attempt to make a change use an ad-hoc process with tools that help in gaining some understanding of the existing system, but which do not provide any specific support

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... or reasoning about, implementing, or analyzing the modification. This thesis presents the Behavioural Concern Modelling (BCM) approach and tool that provide direct support for a systematic approach to modification tasks. This approach helps a developer create a partial, abstract, grounded behavioural model of a concern or concerns. The model is grounded in that the relationship between the model and the code is explicit: A developer describes which code contributes to each part of the model. The examples described use a finite state machine as a model and show how the approach can help a developer capture a concern, reason about design options, and implement modifications. ii