This two-volume set LNAI 14692–14693 constitutes the thoroughly refereed proceedings of the 21st International Conference on Engineering Psychology and Cognitive Ergonomics, EPCE 2024, held as part of HCI International 2024,held 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 papers included in the HCII-EPCE two-volume set were organized in topical sections as follows:Part I: Cognitive Processes and Performance in High-Stress Environments; Decision-Making Support and Automation. Part II: Engineering Psychology and User Experience; Human Factors in Aviation.