We discuss the observational requirements for future x-ray planetary and astrophysics missions and present preliminary laboratory results from our compound semiconductor program. The detectors used in the tests were simple monolithic devices, which are used in conjunction with a detailed material science and technology developmental program intended to produce near Fano limited, pixilated hard X-ray detectors. In practical terms, this means producing active arrays, comprised of over 103 pixels each being of order 100 microns in size, with spectral resolving powers, E/(Delta) E > 20 at 10 keV and high quantum efficiencies over the energy range 1 to 200 keV. Four materials are currently under study--GaAs, HgI2, TlBr and CdZnTe. In the cases of GaAs and CdZnTe, the detector energy resolution functions are approaching the Fano limit.