Aperture synthesis is an attractive alternative to conventional passive microwave imaging systems for remote sensing applications. Whenever a high spatial resolution is desired or the imaging process should work like an optical camera or when microwave remote sensing in the near field has to be distance adaptive, conventional systems cause a lot of problems. However, aperture synthesis for practical applications requires a lot of single receivers and correlators. This can cause other problems because of the large number of possible imperfections. To get an estimation of such effects, the imaging expressions for the aperture synthesis principle are developed for the near- and far-field conditions of a real system. To support the theory and to get an estimation of the feasibility of this imaging method, a simple experimental system is described and experimental results of high spatial resolution for the near and far field are shown. For the near-field case a special reconstruction algorithm was developed and is described theoretically and validated by the experiments. A discussion of the temperature resolution of aperture synthesis concludes the paper. ¿ 1998 American Geophysical Union