This book constitutes the refereed proceedings of the 21st Ada-Europe International Conference on Reliable Software Technologies, Ada-Europe 2016, held in Pisa, Italy, in June 2016. The revised 12 full papers presented together with one invited paper were carefully reviewed and selected from 28 submissions. They are organized in topical sections on concurrency and parallelism, testing and verification, program correctness and robustness, and real-time systems.