Mobility is, as the name suggests, the defining characteristic of mobile computing and the primary differentiator between it and other computer usage paradigms. Traditionally, computers were used in what may be termed a static context. However, when computers are used in a mobile context, a number of difficulties that challenge traditional assumptions emerge. Not least amongst these are those difficulties that arise in delivering a service that is relevant and consistent with the situation in which the end-user find themselves. Should a person be waiting at a bus stop, he or she does not wish to go online and browse a bus timetable. Rather, he or she wishes to know when the next bus will stop at his or her particular stop. Thus location and time would be fundamental to the provision of such a service. Capturing time provides no major difficulties. However, identifying the physical location of a service subscriber may prove problematic. In this review, we summarize some of the key technologies that enable the position of a mobile computer user to be determined.