Present state of X-ray investigations of non-flaring actlve regions is briefly revlewed, It is stressed that it Is very Important to háve simultaneous records of high-quality X-ray spectra and high resolutlon X-ray images. The spectra allow to derive reliahle temperature distrlbutions (differential emission measure) of the coronal plasma. When combined with the X-ray images, it allows to investigate the rate and spatial distrlbution of the energy release (heating of the active region corona), Thls, In tum, provides an important possibiility to test empirically the theories of actlve region heating. These large potential possibillties of active region investigatlon are far from being exhausted and they should be used more efficiently in future Investigations.