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 determine how much gas can be adsorbed at different pressures and temperatures. This information is crucial for understanding the properties of materials in fields like chemistry and materials science. The BET Theory extends the Langmuir adsorption theory by considering multilayer adsorption, where gas molecules can form multiple layers on the surface. By analyzing the amount of gas adsorbed at various pressures, scientists can calculate the specific surface area, which is essential for applications in catalysis, filtration, and drug delivery.