The existence of functionally diverse and plastic β cells in islets of Langerhans has been reported since the 1980s. Recently, high-resolution technologies have advanced our understanding of β-cell heterogeneity and plasticity. Here, we define plasticity broadly as dynamic changes in cellular phenotypes and heterogeneity as differences in cellular behaviors. Individual β cells react differently to environmental challenges and act together to maintain β-cell mass and glucose homeostasis within a narrow range of 70-140 mg/dL. During the progress of diabetes, this elaborate balance is disrupted, and a lack of β-cell compensation leads to dysregulated blood glucose. In this chapter, we assess β-cell stress that instigates increased β-cell heterogeneity and adaptive β-cell responses such as proliferation, dedifferentiation, maturity, and insulin secretion. We also discuss the maturity, electrical activity, and insulin secretion of well-characterized β-cell subgroups. Finally, we touch upon the plasticity of other non-β pancreatic cells and their cooperation with β cells to maintain homeostasis.