We have used Scanning Tunneling Microscopy (STM) to study dopants, point defects, and interfaces in compound semiconductors after cleavage in Ultra‐High‐Vacuum. Features due to SiGadonors and ZnGaacceptors in the STM images of n+GaAs and p+GaAs have been identified and characterized on an atomic scale. For the first time, we found the spherical and triangle‐shape delocalized SiGadonor and ZnGaacceptor features, respectively. In heavily doped n+GaAs, Ga vacancies were observed that produce characteristic dark delocalized spherical features. The Ga vacancies tend to locate close to SiGadonors. We imaged an antisite defect in n+GaAs. High resolution STM spectroscopic images revealed very rich detail of the electronic structures of that site. Isoelectronic In and Ga elements in In0.2Ga0.8alloy were also identified. This allowed us to investigate the interfaces of GaAs/In0.2Ga0.8As/GaAs strained layer quantum wells atom‐by‐atom. Segregation of In at the In0.2Ga0.8As/GaAs interface and clustering inside In0.2Ga0.8As were studied.