The reactivity-regulation model suggests that the origins and maintenance of shyness results from relatively high levels of reactivity in combination with relatively low levels of regulation. Although this model has received some empirical support, there are still issues regarding directionality of the relations among variables and a dearth of studies examining the joint influence of reactivity and regulation on the prospective development of shyness. Using a longitudinal design, we first examined whether the relations among reactivity, regulation, and shyness were unidirectional or bidirectional in a sample of 1284 children (49.8% female, 84.1% White; mean parental education fell between associate degree/diploma and undergraduate degree) assessed annually across three waves from late childhood and early adolescence (M[subscript age] = 10.72 years) to adolescence (M[subscript age] = 12.42 years) and then examined whether reactivity and regulation interacted to influence the development of shyness over time. At Wave 1, shyness was related to higher levels of reactivity and lower levels of regulation at Wave 2, but neither reactivity nor regulation at Wave 1 predicted shyness at Wave 2. At Wave 2, shyness predicted greater reactivity at Wave 3, but shyness at Wave 3 was only predicted by lower levels of regulation at Wave 2. Contrary to the reactivity-regulation model of shyness, we found that relatively high levels of reactivity and low levels of regulation predicted a steep decrease in shyness over 3 years. These results are discussed in the context of the socioemotional difficulties experienced by shy individuals and demonstrate the importance of empirically evaluating long-standing models of personality development.