Whatever the field of human activity-domestic or scientific, work or leisure-it is likely that some knowledge of the behaviour of electrical circuits is required to keep the processes moving, the wheels turning. In many cases, a knowledge of Ohm's law may suffice. In others, an understanding of more complex relationships may be necessary. In this book an attempt is made to provide, in a concise manner, an introduction to the main methods of treating electrical networks, whether they be carrying direct (de) or alternating (ac) electrical currents. Clearly, the range of possible circuits is vast so that the simplifications which are demonstrated in the pages that follow are of great importance to the student. However, to gain the fullest benefit from such a concise presentation, the student must devote some time to the exercises which are provided in Appendix B. The units used throughout the book are those of the International System (or SI). The various quantities which are introduced-such as current and potential and resistance-are summarized in Appendix A together with the symbols used to represent them, the unit associated with each quantity and the formula used to derive that unit from four fundamental or MKSA units.