Communication: The characterization of a Langmuir monolayer at a water surface by means of second-harmonic microscopy (SHM) is reported. Details of the technique and the theory are presented in the following article. The application described here illustrates the main advantages of SHM: it provides quantitative information on the microscopic level and reveals additional information compared to linear microscopy techniques. It is demonstrated to open up new perspectives for the development and improvement of polar organic materials, starting from polar monolayers at an interface.