This book constitutes the proceedings of the 15th International Conference on Tests and Proofs, TAP 2021, which was held as part of Software Technologies: Applications and Foundations, STAF 2021, and took place online during June 12-25, 2021.The 6 full papers included in this volume were carefully reviewed and selected from 10 submissions. They were organized in topical sections on learning, test resource allocation and benchmarks and on testing.