Background and Purpose GABAA-α5 subunit-containing receptors have been shown to play a key modulatory role in cognition and represent a promising drug target for cognitive dysfunction, as well as other disorders. We describe the preclinical and clinical profile of basmisanil, a potent and highly selective negative allosteric modulator (NAM) of GABAA α5 receptors. Experimental Approach In vitro assays assessed binding and functional selectivity. In vivo occupancy studies measured target engagement. Effects on cognition were tested in rats (Morris water maze) and non-human primates (NHP; object retrieval) and potential side effects (anxiety and proconvulsant) were tested in rats. In healthy volunteers, target engagement and modulation of neuronal network activity were assessed using PET and EEG. Key Results Basmisanil bound to recombinant human GABAA-α5 receptors with 5 nM affinity and more than 90-fold selectivity versus α1, α2, and α3 subunit-containing receptors. Basmisanil inhibited GABA-induced currents at GABAA-α5 yet had little or no effect at the other receptor subtypes. In vivo, basmisanil demonstrated dose-dependent target engagement in rats. Basmisanil attenuated diazepam-induced spatial learning impairment in rats and improved executive function in NHPs. At these efficacious plasma concentrations, basmisanil had no anxiogenic and proconvulsant effects. In healthy volunteers, PET showed target engagement and established the plasma exposure to receptor occupancy relationship. Basmisanil modulated brain function reflected in characteristic changes of EEG spectral power. There were no serious adverse events. Conclusion and Implications Basmisanil is a highly potent and selective GABAA α5 receptor NAM with good safety and tolerability allowing for clinical testing in multiple brain disorders.