This chapter extends the material presented in the chapter on inductors and introduces the concept of magnetic circuits. Basic applications are also illustrated. Here we shall take a closer look at the parameters of magnetic circuits such as permeability and flux density, and introduce new parameters including reluctance and magnetizing force. We shall also examine a key graphic that helps define the magnetic properties of materials, namely the $$BH$$ curve. In the process we shall define new terms including coercivity and retentivity, and introduce the concept of hysteresis.