β-Cell cycle progression and proliferation are critical to maintain β-cell mass in adult mice. Of the cell cycle inhibitors, p27Kip1 is thought to be the primary modulator of the proliferative status in most cell types. p27 plays a role in β-cell adaptation in genetic models of insulin resistance. To study the role of p27 in β-cells during physiological conditions and at different stages of β-cell differentiation, we studied mice deficient of or overexpressing p27. Experiments in p27-deficient mice showed improved glucose tolerance and hyperinsulinemia. These changes were associated with increased islet mass and proliferation. The experiments overexpressing p27 in β-cells were performed using a doxycycline-inducible model. Interestingly, overexpression of p27 for 16 weeks in β-cells from adult mice had no effect on glucose tolerance, β-cell mass, or proliferation. In contrast, induction of p27 expression during β-cell development or early neonatal period resulted in severe glucose intolerance and reduced β-cell mass by decreased proliferation. These changes were reversible upon discontinuation of doxycycline. These experiments suggest that p27 is a critical molecule for β-cell proliferation during β-cell development and early postnatal life but not for maintenance of adult mass.