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.

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.