Brewster angle microscopy (BAM) is a highly surface-sensitive, noninvasive technique used to study the reflectivity and lateral domain structures of Langmuir monolayers at the air–water interface. This experimental system uniquely allows for the control of lateral compression while simultaneously measuring surface pressure and visualization of domains. The primary advantage of BAM over other methods is its ability to directly measure in situ the intrinsic properties of samples without requiring chemical modifications, external probes or further manipulations like transfer to a solid support. This chapter outlines the basic theory of BAM and introduces a new method for determining the refractive index of the monolayer, a key parameter often missing in data analysis. By combining the refractive index with reflectivity measurements, it becomes possible to calculate the thickness of films as a function of lateral packing in lipid, protein, and lipid/protein films.