This Special Issue follows the 31st IFIP International Conference on Testing Software and Systems (IFIP-ICTSS). IFIP-ICTSS has become a traditional event of the WG 6.1 working group of the International Federation for Information Processing (IFIP). The conference was held in Paris, France, from October 15 to October 17, 2019. IFIP-ICTSS is a series of international conferences addressing conceptual, theoretical, and practical problems of testing software systems, including communication protocols, services, distributed platforms, middleware, embedded and cyber-physical systems, and security infrastructures. It is a forum for researchers, developers, testers, and users. Its goal is to review, discuss, and learn about new approaches in the field of testing of software and systems. The topics of interest include new concepts, theories, methodologies, tools, and experience reports. The authors of the papers accepted to ICTSS 2019 were invited to submit an extended version to this Special Issue. However, the call for papers was also open to external contributions.