This invention relates to heteroaryl substituted fused bicyclic heteroaryl compounds, such as heteroaryl substituted imidazopyridines, imidazopyrazines, imidazopyridizines, imidazopyrimidines, and imidazothiazoles, which may be described by Formula I or Formula II: ##STR1## The invention is particularly related to such compounds that bind with high selectivity and high affinity to the benzodiazepine site of GABA_A receptors. This invention also relates to pharmaceutical compositions comprising such compounds and to the use of such compounds in treatment of certain central nervous system (CNS) diseases. Processes for preparing compounds of Formula I and Formula II are disclosed. This invention also relates to the use of benzimidazoles, pyridylimidazoles and related bicyclic heteroaryl compounds of Formula I or Formula II in combination with one or more other CNS agents to potentiate the effects of the other CNS agents. Additionally this invention relates to the use such compounds as probes for the localization of GABA_A receptors in tissue sections.