Several serious neurological and psychiatric disorders are associated with cognitive impairment. Thus, drugs targeting the cognitive symptoms of these diseases would be expected to provide significant benefits to these patient populations. A growing body of preclinical evidence has supported the use of serotonin 5-HT6 receptor antagonism as a promising mechanism for treating cognitive dysfunction. Following the discovery of a new generation of 5-HT6 receptor ligands with improved physicochemical properties, leading to superior brain penetration, additional support for the 5-HT6 hypothesis has been provided by clinical trial data for three molecules in drug development.