This chapter provides a review of thermodynamics of liquid films. In particular, film thermodynamics based on the film excess convention is presented. Both the fundamental equation and mechanical equilibrium condition are similar to those of a surface phase in the well-known Gibbsian thermodynamics of surfaces. Experimental techniques for measuring film tension are also reviewed; in particular, Axisymmetric Drop Shape Analysis-Profile (ADSA-P) is introduced. ADSA-P is the only existing technique which is not restricted to spherical drops or bubbles. With ADSA-P, film tension can be measured under both static and dynamic conditions. As a demonstration of ADSA-P, film tension and film rupture of alkanes (hexadecane, tetradecane, and dodecane) at the air-water interface are presented. The film tension measured is found to be equal to the sum of alkane-water interfacial tension and alkane surface tension for hexadecane and tetradecane films. In the case of dodecane films, a slight difference between the film tension and the sum of two surface (interfacial) tensions is observed and can be attributed to the interactions between the two surfaces of the film, which may be quantified by disjoining pressure.