Film balance studies at an air/water interface provide a useful method for the study of processes and reactions involving proteins. It has the great advantage that the kinetics of a process or reaction can be measured by the area change at constant interfacial pressure while precisely monitoring important variables such as the area occupied per molecule. An example of how the film balance may be used to separate physical processes in protein monolayers and to measure their kinetics is illustrated using P-lactoglobulin. The use of multi-compartment troughs is particularly valuable for studying protein reactions such as those of enzymes. These simple techniques can be used effectively to probe questions about fundamental aspects of protein behaviour which are still controversial, e.g., the question of reversibility of protein adsorption, the applicability of the Gibbs adsorption equation, the role of an interfacial energy barrier in adsorption and the retention of secondary and tertiary structure at interfaces. In addition, studies by the film balance can be used as a basis for elucidating the role of proteins in industrial processes (stabilization of emulsions, foams and suspensions), biological and biomedical areas (biomembranes, enzyme reactions) and in new technologies utilizing Langmuir-Blodgett layers.