Artificial Intelligence and Reuse -- The Unexpected Blocking of Code Understanding in AI-Based Code Summarization: Observations and Concerns from a Study on Cross-Project Learning Performance -- On Modularity of Neural Networks: Systematic Review and Open Challenges -- Generative AI for Code Generation: Software Reuse Implications -- Variability and Reuse -- Complexity of In-Code Variability: Emergence of Detachable Decorators -- Design Pattern Representation and Detection based on Heterogeneous Information Network -- An Ontology-based Representation for Shaping Product Evolution in Regulated Industries -- Adaptation and Reuse -- Assessing Reflection Usage with Mutation Testing Augmented Analysis -- Using energy consumption for self-adaptation in FaaS -- Code Reuse -- Using code from ChatGPT: Finding patterns in the developers’ interaction with ChatGPT -- Evaluating the Reusability of Android Static Analysis Tools -- The current status of open source ERP systems: a GitHub analysis.

This volume constitutes the refereed proceedings of the 21st International Conference on Software and Systems Reuse, ICSR 2024, held in Limassol, Cyprus, during June 19–20, 2024. The 9 full papers presented together with 2 short papers were carefully reviewed and selected from 25 submissions. They focus on the following topics: artificial intelligence and reuse; variability and reuse; adaptation and reuse; and code reuse.