This book constitutes the refereed proceedings of the 18th International Conference on Augmented Cognition, AC 2024, held as part of the 26th HCI International Conference, HCII 2024, which took place in Washington, DC, USA, during June 29-July 4, 2024. The total of 1271 papers and 309 posters included in the HCII 2024 proceedings was carefully reviewed and selected from 5108 submissions. The AC 2024 proceedings were organized in the following topical sections: Part I: Understanding cognitive processes and human performance; advancing cognitive abilities and performance with augmented tools; Part II: Advances in augmented cognition technologies; applications of augmented cognition in various contexts.