A simple second-harmonic microscope for in situ and real-time investigation of interfaces and ultrathin films is described. A spot on the interface is illuminated with a laser. The frequency-doubled light is used for imaging. The technique is surface specific. A single laser pulse is sufficient to create an image. Scanning of the sample with a sharply focused laser beam is not required. Harmonic imaging reveals more information on surface order and symmetry than is possible with linear optical microscopy experiments. Polarization microscopy with uncrossed polarizers is utilized as a linear optical technique. This simple method provides superior resolution compared with Brewster angle microscopy.