This book came about as a result of a course taught in the Physics Department at the University of Wisconsin in Madison. The students in the course are mostly Juniors majoring in physics or engineering, and it is assumed they have had a good introductory physics course and a course in calculus. No previous knowledge of electronics is assumed. The more advanced mathematical techniques (differential equations, complex variables, and Fourier analysis) are explained in some detail where they are first encountered, and students with no previous exposure to these topics should be able to understand them without great difficulty. The book divides naturally into two parts. The first part (Chapters I to 5) covers linear circuits. The second part (Chapters 6 to 12) covers nonlinear circuits.