This book provides a theoretical discussion of pulse width modulation (PWM) in power electronic inverters. Pulse width modulation is widely used for the frequency control of speed of ac motors, the design of uninterruptible power supplies (UPS) as well as the integration of renewable energy sources into existing power grid systems. PWM technique is based on approximation of sinusoidal waveforms by sequences (trains) of rectangular pulses whose widths are properly modulated. This width-modulation results in the suppression of low order harmonics at the expense of amplification of high order harmonics which are suppressed by energy-storage elements in load circuits. The discussion covers various PWM techniques with a focus on the optimal time-domain PWM techniques proposed by the authors.