For wireless communication engineers, it is important to have solid fundamental knowledge of noise and how to minimize it by stabilizing the incoming/outgoing waves.  This introductory text of frequency stability offers discussion of the noise from the practical and theoretical points of view, proceeding with investigation of frequency and time fluctuations in resonators, and continue with stability of both of standard and practical microwave oscillators. Finally, the author discusses noise properties of building circuit blocks introducing a chapter on time domain properties and their relations with noise spectral densities. A special chapter is dedicated to the design and properties of the Phase Locked Loops. They are very important for frequency synthesizers which influence every day communications of millions and millions of people.