Monolayers of amphiphilic lipids serve as a proxy for lipid membranes in studies of membrane structure and membrane-protein interactions, since they are significantly more straightforward to study with wide range of spectroscopic and microscopic methods than lipid bilayers. Moreover, lipid monolayers form functionally essential structures that line the alveoli in the lungs and cover the surfaces of the eyes. In the lungs, a pulmonary surfactant (PS) monolayer covers the alveolar liquid, preventing the collapse of the alveoli during exhalation. In the eyes, a tear film lipid layer (TFLL) consists of a monolayer that separates the tear fluid from the non-polar wax layer of the TFLL, thus helping the latter spread rapidly between eye blinks. Both PS and TFLL are compositionally complex, likely to optimize their mechanical behavior under dynamic conditions. Notably, this behavior depends on the subtle balance of forces at the liquid-air interface.