This chapter reviews the basic algebraic and geometrical properties of complex numbers, and shows the power of complex numbers as tools for solving a number of basic electrical engineering problems. The addition or subtraction rules for complex numbers are geometrically translated through the parallelogram rules for the addition and subtraction of vectors. Alternately, a complex number as an argument in the plot command is interpreted by MATLAB to mean: take the real part of the complex number to be the x-coordinate and the imaginary part of the complex number to be the y-coordinate. This alternative syntax works when the imaginary part of the complex number is not zero. The main advantage of writing complex numbers in polar form is that it makes the multiplication and division operations more transparent, and provides a simple geometric interpretation to these operations.