At the end of 1993 there were about 300 million users of personal computers (PCs) and in 1997 their number was estimated at almost 2 billion. The mass diffusion of the PC since the middle of the 1970s has spread the need for computer literacy, supermarkets list computers as ordinary commodities, and nowadays everyone is supposed to be able to choose and work with a computer, without having any special competence. A PC is just slightly more difficult to use than a car, or domestic white or brown goods, not least because the latter too have become computerised. Of course, each device has a number of features that one should be informed about if one aims to distinguish between the different models, choose more wisely according to one’s own needs and employ them to their best. Nevertheless, it is also true that the products on sale are all subject to the same marketing strategies, so the price/performance ratio is hardly ever a source of big surprises. Consequently, if one buys a PC of a decent brand in the nearest shop, with a dash of common sense and an eye to one’s wallet rather than to the latest advertisements, one is unlikely to make a big mistake. All this is to reassure the computer-terrified. On the other hand, if you wish to buy a better machine, obtain more for less, get the most from your PC and aim at high performance you need some essential information about its basic features. Moreover, and more importantly, if you are interested in the philosophical issues arising from the information revolution you cannot avoid having some elementary knowledge of the technology in question. The task of the present chapter is to provide this minimal background. It is far from being a technical introduction to computing, which would require a whole volume by itself (see for example Boolos and Jeffrey 1989; Brookshear 1989 and 1997; Cohen 1997; Dewdney 1996; Hopcroft and Ullman 1979; Lewis and Papadimitriou 1998; Minsky 1967; Moret 1998; Savage 1998; Sipser 1997; Taylor 1997). It is only a guide to what one needs to know to be able to take advantage of the technology in question and understand more critically some of the points I shall make in the following chapters.