This book constitutes the proceedings of the 45th International Conference on Conceptual Modeling, ER 2026, held in St. John's, NL, Canada, during October 5–8, 2026.

The 20 full papers and 13 short papers included in this book were carefully reviewed and selected from 104 submissions. They are organized into the following topical sections: Conceptual Modeling for Data-Intensive and Regulated Domains; LLMs and AI for Conceptual Modeling; Responsible AI, Regulation and Security; Foundational and Reference Ontologies; Business Process Discovery and Optimization; Modeling Concepts and Modeling Practice; Advances in Ontology Engineering; Requirements Engineering and Modeling Foundations; LLMs and Multi-Agent Systems; Process Modeling and Compliance.