Active thought involves a complex set of thinking skills that allows one to transcend routine-bound behaviour to address novel problems. Due to their extraordinary complexity, these cognitive processes remain poorly understood and remarkably resistant to quantification. This thesis aims to improve the understanding and clinical assessment of these important abilities. The first literature review revealed some degree of fractionation within the frontal lobes, with different areas supporting different cognitive processes involved in active thought. In the second literature review, evidence from focal lesions studies revealed that few existing clinical tests have been demonstrated to detect impaired active thinking following frontal lesions. Three retrospective focal lesion studies were conducted with the aim of characterising fluid intelligence (Gf) and its relationship with the brain. Contrary to predictions arising from an influential theory of Gf, frontal patients’ performance on the Weigl Colour-Form-Sorting test could not be explained by reduced Gf. Moreover, performance on different tests of Gf was underpinned by different cognitive processes, supported by different lateralised frontal areas. Three further retrospective investigations sought to improve understanding of the relationship between the brain and the cognitive processes involved in inductive reasoning and verbal generation. Unexpectedly, performance on the Brixton Spatial Anticipation Task, a test of inductive reasoning, was unimpaired in frontal patients, limiting the conclusions that could be drawn about inductive reasoning but highlighting the Brixton’s insensitivity to frontal lesions. Analysis of quantity and quality of phonemic fluency responses revealed that verbal generation is underpinned by a set of well-localised left frontal regions and white matter tracts, with normal performance relying upon a group of specific cognitive mechanisms. These findings suggest that active thought is supported by a diverse range of executive processes, associated with different frontal areas. Thus, its clinical assessment requires several tests, capable of assessing its distinct underlying cognitive processes.