This chapter reviews the principle domains assessed by clinical neuropsychologists when conducting a cognitive assessment and the utility of assessment in diagnosis and clinical management. It provides an overview of methods for estimating prior functioning, the non-specific nature of patients’ subjective neurological complaints, validity and base rate issues, and some of the difficulties and complicating issues that arise when interpreting neuropsychological data. Those interested in succinct summaries of clinical presentations and “bedside” measures may find Hodges [1] and Larner [2] useful further reading, and those interested in detailed reviews of cognitive tests can do no better than to consult Lezak, Howieson, Bigler and Tranel’s [3] authoritative text.