The total of 1276 papers and 275 posters included in the 40 HCII 2021 proceedings volumes was carefully reviewed and selected from 5583 submissions.

The posters presented in these four volumes are organized in topical sections as follows: Part I: user experience design and evaluation; visual design and visualization; data, information and knowledge; interacting with AI; universal access, accessibility and design for aging. Part II: multimodal and natural interaction; perception, cognition, emotion and psychophysiological monitoring; human motion modelling and monitoring; IoT and intelligent living environments. Part III: learning technologies; HCI, cultural heritage and art; eGovernment and eBusiness; digital commerce and the customer experience; social media and the metaverse. Part IV: virtual and augmented reality; autonomous vehicles and urban mobility; product and robot design; HCI and wellbeing; HCI and cybersecurity.