This book constitutes the refereed proceedings of the 18th International Conference on Web Engineering, ICWE 2018, held in Cáceres, Spain, in June 2018. The 18 full research papers and 17 short papers presented together with 2 practice papers, 6 demonstration papers, and 5 tutorials were carefully reviewed and selected from 103 submissions. The papers cover research areas such as Web application modeling and engineering; Web infrastructures and architectures; execution models; human computation and crowdsourcing applications; Web application composition and mashups; Social Web applications; Semantic Web applications; Web of Things applications; big data and data analytics; and security, privacy, and identity.