In this chapter, we provide a basic introduction to the measurement of mood. After decades of neglect, mood emerged as a seminal concept within psychology during the 1980s, and its prominence has continued unabated ever since. Indeed, a PsycINFO database survey during the 5-year period from 2001 to 2005 generated 10,527 references with the keyword mood; the same search conducted over the 5-year period from 2006 to 2010 yielded a total of 15,065 relevant references. Our survey of this rapidly expanding literature is organized into three broad sections. First, because good instruments obviously should assess the basic constructs in a domain, we examine contemporary thinking and research regarding the underlying structure of mood. Second, we briefly describe and evaluate many of the most important measures in this area. Finally, we discuss several general issues related to the reliability and construct validity of mood measures. Before turning to these other matters, however, we first need to define the domain itself. What exactly is a mood, and how does it differ from the related concept of emotion? Moods can be defined as transient episodes of feeling or affect (Watson, 2000a). As such, moods differ from emotions in several important ways (see Larsen, 2000; Watson, 2000a, 2000b); we restrict ourselves here to three key differences that have important implications for measurement. First, mood research focuses primarily—-indeed, almost exclusively—on subjective, phenomenological experience. In contrast, emotions classically have been viewed as multimodal psychophysiological systems, with at least four differentiable components:(1) the subjective (eg, feelings of fear and apprehension);(2) the physiological (eg, activation of the sympathetic nervous system);(3) the expressive (eg, the facial expression of fear); and (4) the behavioral (eg, flight away from danger; Watson, 2000a; Watson & Clark, 1992). Thus, in sharp contrast to emotion research, mood measurement essentially involves the assessment of subjective feelings, without any systematic consideration of these other components.

Second, emotions tend to be extremely brief, lasting perhaps only a few seconds (Izard, 1991; Larsen, 2000). One occasionally observes prolonged emotional states, but these extended reactions tend to be dysfunctional manifestations of psychopathology (see Clark & Watson, 1994; Watson, 2000a). In contrast, moods typically are much longer in duration. For example, whereas the full emotion of anger might last for only a few seconds, an annoyed or irritable mood may persist for several hours, or even for a few days. Because of their longer duration, moods are more easily linked to long-term individual differences in temperament and personality. Indeed, some prominent measures in this area—such as the Multiple Affect Adjective Checklist–Revised (MAACL-R; Zuckerman & Lubin, 1985) and the Positive and Negative Affect Schedule–Expanded Form (PANAS-X; Watson & Clark, 1999)—contain alternative versions that permit one to assess either (a) short-term fluctuations in current mood or (b) long-term individual differences in trait affect. Accordingly, we consider both state and trait affect in our review. Third, the concept of mood subsumes all subjective feeling states, not simply those experiences that accompany classical, prototypical emotions such as fear and anger. This has caused some confusion in the literature; writers periodically have criticized mood measures for