This book constitutes the refereed proceedings of the 43rd IFIP WG 6.1 International Conference on Formal Techniques for Distributed Objects, Components, and Systems, FORTE 2023, held in Lisbon, Portugal, in June 2023, as part of the 18th International Federated Conference on Distributed Computing Techniques, DisCoTec 2023.The 13 regular papers and 3 short papers presented in this book were carefully reviewed and selected from 26 submissions. They cover topics such as: concurrent programming; security; probabilities, time and other resources; and model-based testing and petri nets.