Human-computer interaction (HCI) is concerned with designing interactions between human activities and the computational systems that support them, and with constructing interfaces to afford those interactions. Interaction between users and computational artefacts occurs at an interface that includes both software and hardware. Thus, interface design impacts the software life-cycle in that it should occur early; the design and implementation of core functionality can influence the user interface–for better or worse. Because it deals with people as well as computational systems, as a knowledge area HCI demands the consideration of cultural, social, organizational, cognitive and perceptual issues. Consequently, it draws on a variety of disciplinary traditions, including psychology, ergonomics, computer science, graphic and product design, anthropology and engineering.