The most popular definition of job satisfaction was supplied by Locke (1976), who defined it as "… a pleasurable or positive emotional state resulting from the appraisal of one's job or job experiences" (p. 1304). There are many possible influences on how favorably one appraises one's job, and numerous theories of job satisfaction have attempted to delineate these influences. Empirical evidence, however, has suggested only one clear aspect of the work environment that consistently influences job satisfaction – the intrinsic challenge present in the work itself. This leads to the general principle that will be the focus of this chapter – that mentally challenging work is the key to job satisfaction. Thus, the most effective way an organization can promote job satisfaction of its employees is to enhance the mental challenge in their jobs, and the most consequential way most individuals can improve their own job satisfaction is to seek out mentally challenging work.