There are, at present, few books in this field. Most books on computer design cover the more elementary methods, and some go into detail on one or two more ambitious units. Space does not allow more. In this text the aim has been to fill this gap in the literature. In selecting the topics to be covered, there have been two main aims: first, to deal with the basic procedures of arithmetic, and then to carry on to the design of more powerful units; second, to maintain a strictly practical approach. The number of mathematical formulae has been kept to a minimum, and the more complex ones have been eliminated, since they merely serve to obscure the essential principles.