Oscillators are the building blocks of some very important circuits such as Phase Locked Loops, PLLs, frequency synthesizers, local signals for Mixers, and clocks for digital ICs. Moreover, injection locked oscillators could be suitable approaches to multiphase signals generation, frequency division or multiplication, and synthesis. Nowadays, there have been some brilliant methods to realizing the oscillators, and frequency generation, or (injection) locked them to a reference. In addition, reaching frequencies near or beyond millimeter wave range (sub-terahertz and terahertz frequencies) is rather hard but practically intriguing area of focus. Working on harmonics to produce higher frequency signals have evolved over time. Reducing the phase noise of generated signals has also been an on demand subject. This special issue concentrates on advances on oscillators and frequency generators, both for low frequency or (ultra) high frequency range. Every improvement or analysis in this filed is considered. For new structures have been proposed for oscillators it is important to provide theoretical approach to study the parameters such as amplitude of first harmonic, amplitude of higher harmonics, frequency, phase noise, and jitter. Moreover, it is acceptable to propose new methods to generate low or high frequency range oscillations, new structures of oscillators with a better performance, or new proposals to improve the oscillator performance are important for this especial issue.