This book constitutes the proceedings of the 12th International Conference on Verification and Evaluation of Computer and Communication Systems ( VECoS 2018) held at Grenoble, France, in September 2018. The 11 full papers in this volume, presented together with one abstract and two invited papers, were carefully reviewed and selected from 23 submissions. The aim of the VECoS conference is to bring together researchers and practitioners in the areas of verification, control, performance, and dependability evaluation in order to discuss state of the art and challenges in modern computer and communication systems in which functional and extra-functional properties are strongly interrelated. Thus, the main motivation for VECoS is to encourage the cross-fertilization between various formal verification and evaluation approaches, methods and techniques, and especially those developed for concurrent and distributed hardware/software systems.