A Langmuir monolayer is a one-molecule thick insoluble layer of an organic material spread onto an aqueous subphase. Traditional compounds used to prepare Langmuir monolayers are amphiphilic materials that possess a hydrophilic headgroup and a hydrophobic tail. Since the 1980s a large number of other materials have been employed to produce Langmuir monolayers, some of which are semi-amphiphilic, including macromolecules such as polymers. Langmuir monolayers are extensively studied for the fabrication of Langmuir-Blodgett (LB) films, which are formed by transferred monolayers on a solid substrate.

Compare with the phospholipid lipid bilayer structure of biological membraness.