Why, and how, do we differ from one another? Does parenting affect children's abilities and personalitis? Or are they largely genetically determined? If personality and intelligence can be assessed, how exactly do psychometric tests work? And how useful is the information they provide? This text seeks to offer an integrated and objective presentation of this field of psychology, covering human personality, ability, mood and motivation. The author shows how psychological tests can be used to measure individual differences and provides background in psychometric theory (including factor analysis and item-response theory) to enable the reader to understand how tests are constructed, evaluated and used. Specifically designed for psychology students taking a course on personality or individual difference, the text contains self-assessment exercises (and answers) in each chapter.