In the framework of past and future x-ray missions the SAX satellite, to be launched in March 1996, stands out for its very wide spectral coverage from 0.1 to 200 keV, with well balanced performances of the low energy and high energy instrumentation. The sensitivity of the scientific payload will allow the exploitation of the full band of SAX also for weak sources (1/20 of 3C273), opening new perspectives in the study of spectral shape and variability of several classes of objects. In this paper we briefly describe the main aspects of the mission, with particular regard to its scientific capabilities.