Nanoelectromechanical systems (NEMS) are microelectromechanical devices (MEMS) scaled down to nanometer range. NEMS or nanomechanical oscillators can be fabricated to achieve high natural resonance frequencies, exceeding 1 GHz. These oscillators are candidates for ultrasensitive mass sensors and frequency determining elements of precision on-chip clocks. The central question of this book is: as the size of the nannomechancial oscillators becomes smaller, how stable can the resonant frequency be? The answers to this seemingly simple question form the subject of phase noise for nanomechanical oscillator. In this book, the full theory of this subject is presented with the introduction of common terminology used in phase noise.