Optical frequency combs are light sources associated with a spectrum composed of many sharp individual lines. The idea to employ them as links between the radio and optical frequency domains (Nobel Prize in Physics, 2005) has changed the world in the areas of optical frequency metrology, precision measurement, and spectroscopic applications. Today these combs are commonly generated from pulsed, passively mode-locked solid-state or fiber lasers, which are however bulky and energy inefficient, limiting their application and implementation possibilities. The realization of these sources in compact devices will dramatically improve their complexity management, cost figure, and power consumption, thus allowing completely new application fields. By working in the quantum optics regime, new sources also enable advanced applications in fields like telecommunication, cryptography, and computing.