Langmuir monolayers at the air/water interface have been used for decades to mimic cell membranes in attempts to determine the mechanisms behind the physiological action of biologically-relevant molecules. In this review, we analyze the vast literature in the area, with the contents organized according to the type of molecules and materials, including peptides, proteins, polysaccharides, a variety of pharmaceuticals, and nanomaterials. The focus is placed on the correlation between the effects induced on the monolayers and the biological activity of the molecules and nanomaterials. Effects observed from these interactions can be coupling or adsorption and penetration of the molecules into the monolayer, which can be expanded, condensed or even disrupted. Changes in monolayer mechanical properties, for example, may be crucial for the biological activity. Whenever possible, we try to identify the forces prevailing in the interaction, which has been made possible with a combination of experimental techniques, including surface-specific spectroscopies, microscopies and rheological techniques, in addition to the traditional surface pressure and surface potential measurements. Overall, the mechanisms are governed by ionic electrostatic forces and hydrophobic interactions. Correlation may be straightforward, as in the cases of positively charged peptides and polymers whose antimicrobial activity is ascribed to electrostatic attraction with the negatively charged microbial membranes. Also general is the importance of hydrophobic interactions for the penetration into the membrane, which can be required for the biological action of, for example, polysaccharides. In other cases, correlation between monolayer properties and the physiological activity cannot be established precisely, as the latter may depend on a multitude of parameters that have not been possible to simulate with a simplified model such as that of a Langmuir monolayer. For nanomaterials, the emphasis is in relating interaction with the monolayers and their possible toxicity. Owing to the relevance of electrostatic and hydrophobic interactions, the effects on monolayers (and indeed toxicity) are found to depend largely on the coating or functionalization of the nanomaterials. (C) 2015 Elsevier B.V. All rights reserved.