The Brunauer-Emmett-Teller (BET) Theory is a scientific method used to measure the surface area of materials, particularly porous substances. It is based on the physical adsorption of gas molecules onto a solid surface, allowing researchers to understand how much gas can be stored or absorbed by the material. This theory expands on the Langmuir adsorption isotherm by considering multilayer adsorption, where gas molecules form multiple layers on the surface. By analyzing the amount of gas adsorbed at different pressures, the BET method provides valuable insights into the texture and porosity of materials, which is essential in fields like materials science and catalysis.