This chapter summarizes some important aspects and approaches to design oscillators. Electronic oscillators are classified according to RC, LC, and crystal. The signal source normally found in any analog-type sine wave generator or a function generator is an RC oscillator. RC oscillators can be further classified as phase shift, Wien bridge, and twin-T. The signal source in a radio frequency (RF) signal generator is normally an LC oscillator. LC oscillators can be further classified into Hartley, Colpitts, and Clapp types. Crystal oscillators produce very accurate and stable frequencies. However, oscillation frequency tends to change with the age and operating temperature of the crystal. A common method of generating RF oscillation is by using a phase-locked loop (PLL). PLLs are commonly found in areas such as communications, wireless systems, digital circuits, and disc drive electronics. The PLL contains four building blocks: a phase detector, a low-pass filter, a voltage-controlled oscillator, and a frequency divider.