One long-standing theoretical model of shyness proposes that the origins and maintenance of shyness are associated with an approach-avoidance motivational conflict (Asendorpf, 1990), such that shy individuals are motivated to socially engage (high approach motivation) but are too anxious to do so (high avoidance motivation). However, this model has not been empirically tested in predicting the development of shyness. In two separate longitudinal studies, we used the Carver and White (1994) Behavioral Inhibition and Activation System (BIS/BAS) scales as a proxy of approach-avoidance motivations and growth curve analyses to examine whether individual differences in these hypothesized motivational tendencies were associated with the development of shyness across 3 years from late childhood to adolescence (Study 1, N = 1284; 49.8% female, M[subscript age]= 10.72, SD[subscript age] = 1.73, M level of parental education fell between associate's degree/diploma and undergraduate degree) and across nearly a decade from emerging adulthood to young adulthood (Study 2, N = 83; 57.8% females, M[subscript age] = 23.56 years, SD[subscript age] = 1.09 years, 92.8% had at least a high school education). Contrary to the approach-avoidance conflict model of shyness, we found that a combination of high BIS/low BAS, not high BIS/high BAS, was associated with relatively higher shyness contemporaneously and across development in both studies. We discuss the processes that might link individual differences in approach-avoidance motivations to the development of shyness in adolescence and young adulthood.