BET Theory, or Brunauer-Emmett-Teller Theory, is a scientific model used to describe the physical adsorption of gas molecules on solid surfaces. It extends the Langmuir Theory by accounting for multilayer adsorption, allowing for a more comprehensive understanding of how gases interact with solids. The theory is particularly useful in characterizing porous materials and determining surface area. The BET equation relates the amount of gas adsorbed to the pressure of the gas, providing a way to calculate the surface area of materials like activated carbon and zeolites. This theory is widely applied in fields such as material science, chemistry, and environmental engineering.