In this article we will examine some important issues related to human-computer interaction (HCI). This will be followed by a discussion of usability and its underlying principles and properties. The dependability of computer systems is intrinsically multi-faceted. Dependable hardware is patently of limited value unless accompanied by dependable software. Neither helps greatly if human interaction with the hardware and software system is fault-prone and the dependable socio-technical performance of an inappropriate task may cause wider damage (MacKenzie, 2000).