Inverted pendulum is an example of naturally unstable system. In order to stabilize an inverted pendulum, a proper controller should be used. One of the famous controllers is PID. PID controller can be analog (consist of several operational amplifiers) or digital. In this book, a microcontroller is used as a digital PID controller. The real PID formula contains a differentials equation. An 8-bit microcontroller is not fast enough to calculate such formula within a short time (less than 10ms). Therefore, the formula should be first simplified; hence, the microcontroller can perform the computation in a tight timing. The actuator for this inverted pendulum is a DC motor. The speed of the motor is controlled using PWM (Pulse Width Modulator).