This volume reflects the pressure to develop useful models and methodologies to study executive behaviour - the ability to update information in working memory in order to control selective attention to formulate plans of action and to monitor their efficient execution. Many models are based on the concept of a single central executive that manges these functions; others propose a number of independent working memory systems that each serve one task or activity but not others.; This book is a collection of essays by active researchers who discuss their own work on the definition of executive or controlled behaviours, and on the relation of these behaviours to specific areas of the frontal cortex. The papers are particularly concerned with logical difficulties that arise in defining these functions that lead, in turn, to methodological difficulties in studying them. In particular, they discuss such problems as the low test-re-test reliability of tasks that have been used to define and explore executive behaviours, the limited validity of these tasks in predicting performance deficits, the poor localization of the changes observed with respect to underlying brain function, and the relation of performance on these tasks to individual difference in performance on measures of global or general intellectual ability such as Spearman's 1927 gf.; The authors discuss their own research on the relations between cognitive function and neuropsychology, on changes in executive competence in conditions such as closed head injuries or dementias that may diffusely affect the whole brain, and on changes in executive function in normal old age.