The three-volume set CCIS 850, CCIS 851, and CCIS 852 contains the extended abstracts of the posters presented during the 20th International Conference on Human-Computer Interaction, HCI 2018, which took place in Las Vegas, Nevada, in July 2018. The total of 1171 papers and 160 posters included in the 30 HCII 2018 proceedings volumes was carefully reviewed and selected from 4346 submissions. The 207 papers presented in these three volumes are organized in topical sections as follows: Part I: interaction and information; images and visualizations; design, usability and user experience; psychological, cognitive and neurocognitive issues in HCI; social media and analytics. Part II: design for all, assistive and rehabilitation technologies; aging and HCI; virtual and augmented reality; emotions, anxiety, stress and well-being. Part III: learning and interaction; interacting with cultural heritage; HCI in commerce and business; interacting and driving; smart cities and smart environments.