This volume constitutes the proceedings of the 12th International Conference on Cloud Computing, CLOUD 2019, held as part of the Services Conference Federation, SCF 2019, in San Diego, CA, USA, in June 2019. The 24 full papers were carefully reviewed and selected from 53 submissions. CLOUD has been a prime international forum for both researchers and industry practitioners to exchange the latest fundamental advances in the state of the art and practice of cloud computing, to identify emerging research topics, and to define the future of cloud computing. All topics regarding cloud computing align with the theme of CLOUD.