Theories of emotion generally posit the synchronized, coordinated, and/or emergent combination of psychophysiological, cognitive, and behavioral components of the emotion system--emotional concordance--as a functional definition of emotion. However, the empirical support for this claim has been weak or inconsistent. As an introduction to this special issue on emotional concordance, we consider three domains of explanations as to why this theory-data gap might exist. First, theory may need to be revised to more accurately reflect past research. Second, there may be moderating factors such as emotion regulation, context, or individual differences that have obscured concordance. Finally, the methods typically used to test theory may be inadequate. In particular, we review a variety of potential issues: intensity of emotions elicited in the laboratory, nonlinearity, between- versus within-subject associations, the relative timing of components, bivariate versus multivariate approaches, and diversity of physiological processes.