This chapter reviews the main fundamental knowledge of foam films. The liquid solution is contained in foam films which meet in Plateau borders and vertexes. The foam film is the most important structural element of foam since it determines foam stability and the foam's main properties. The thermodynamic definition of the thin liquid film shows that the most important factor which determines its properties is the interaction between the two film interfaces, that is, the interactions due to different types of surface forces between the two adjacent phases across the liquid film. The chapter considers the methods of microscopic foam films, in conjunction with the micro-interferometric technique as well as the pressure balance technique. Black foam films can reach an extremely small thickness. When observed under a microscope, they are black since they reflect minimal light when their thickness is below 20 nm.