The main motivation that we had for writing this book is to cover the latest research and results that the first author achieved at THO laboratory in Montpellier (France) for the characterization of the electronic noise of devices used as terahertz sources (emitters or detectors). The idea of the book started with a meeting of scientific course in Montpellier in 2016, where the authors of this book has proposed the contents and the objective form of the book.