This unit presents arguments for why it is vital to include HCI tools, techniques and practices into the design of interactive computer systems. To do this, four viewpoints are taken: business (to show how user-centred design can impact on performance); quality of life (bad HCI can lead to frustration, anxiety and anger); safety-critical (some systems, if badly designed, can kill); and, the legislation/ international standards are relevant. Many interactive computer systems seem to have been developed with little thought of the user: this unit will discuss the possible reasons for this.