Molecular Beam Epitaxy (MBE) is an Ultra-High-Vacuum (UHV)-based technique for producing high quality epitaxial structures with monolayer control. Since its introduction in the 1970s as a tool for growing high-purity semiconductor films, MBE has evolved into one of the most widely used techniques for producing epitaxial layers of metals, insulators and superconductors as well, both at the research and the industrial production level. The principle underlying MBE growth is relatively simple: it consists essentially of atoms or clusters of atoms, which are produced by heating up a solid source such as graphite or a cluster of pure carbon. They then migrate in an UHV environment and impinge on a hot substrate surface, where they can diffuse and eventually incorporate into the growing film. Despite the conceptual simplicity, a great technological effort is required to produce systems that yield the desired quality in terms of material purity, uniformity and interface control. The control on the vacuum environment and on the quality of the source materials allows a much higher material purity, compared to non-UHV-based techniques.