This chapter reviews conceptual issues and definitions of the executive function. It is argued that executive function is a multidimensional construct and it is suggested that subspecialists in Behavioral neurology (BN) & Neuropsychiatry (NP) regard executive function principally as a cognitive domain. Executive function requires the integrated actions of the frontal-subcortical circuits, open-loop connections to other neocortical areas, limbic and paralimbic structures, thalamic nuclei, pontocerebellar networks, modulatory neurochemical projections from mesencephalic and ventral forebrain structures, and the white matter connections within and between all of these areas. As such, executive dysfunction is more accurately understood as dysfunction within or across these networks. The distributed structural and functional anatomy of executive function renders it vulnerable to disruption by many conditions affecting the brain. Finally, the chapter briefly discusses neuropsychological tests and bedside assessments of executive function.