This special issue contains selected papers from the IEEE International Conference on Software Maintenance that took place in Budapest on 26-29 September 2005. From a total of 180 papers submitted, the program committee selected 55 full papers for inclusion in the proceedings. Of these 55 papers, four were extended from their conference version, were reviewed according to the Journal of Software Maintenance and Evolution: Research and Practice reviewing process, and are contained in this special issue. These papers are interesting case studies that deal with characteristic issues of software evolution and/or maintenance.In the paper 'Supporting the analysis of clones in software systems: a case study' by Kapser and Godfrey, the authors explore code duplication and offer a qualitative study into how it actually manifests itself within software systems. They present an in-depth case study of cloning in a large software system that is in wide use, the Apache Web server. They found several interesting types of cloning occurrences, such as cloning hotspots, where a single subsystem comprising only 17% of the system code contained 38.8% of the clones. They also found several examples of cloning behavior that were beneficial to the development of the system, in particular cloning as a way to add experimental functionality.In the next paper 'How do APIs evolve? A story of refactoring' by Dig and Johnson, the authors explore migration of an application to a new API and studied the API changes of four frameworks and one library. They discovered that the changes that break existing applications are not random, but tend to fall into particular categories. Over 80% of these changes are refactorings. This suggests that refactoring-based migration tools should be used to update applications.In the next paper 'Refactoring a legacy component for reuse in a software product line: a case study' by Kolb et al., the authors explore the migration of a legacy software component, which has not been designed for reuse, into a reusable product line component. The component under investigation is responsible for controlling memory usage and compressing and decompressing image data. The improvements of both the component's design and implementation are focused on increasing maintainability and reusability and, hence, suitability for use in a product line. As a result of the analysis and refactoring activities, the documentation and implementation of the component have been considerably improved as shown by quantitative data. Despite a number of changes to the code, the external behavior of the component has been preserved without significantly affecting the performance.
