Research Ne,vs: Photovoltage images of semiconductor surfaces, obtained using a scanning tunneling microscope (STM), can provide valuable information about the electronic properties of the semiconductor interface. The advantages of photo-STM, a technique that enables images of the photocurrent together with the topography of a semiconductor sample to be recorded using a commercial STM, are discussed. For example, direct observation of different physical phenomena with nanometer resolution is possible and space-charge regions can easily be detected.