β-Cells possess inherent mechanisms to adapt to overnutrition and the prevailing concentrations of glucose, fatty acids, and other fuels to maintain glucose homeostasis. However, this is balanced by potentially harmful actions of the same nutrients. Both glucose and fatty acids may cause good/adaptive or evil/toxic actions on the β-cell, depending on their concentrations and the time during which they are elevated. Chronic high glucose dramatically influences β-cell lipid metabolism via substrate availability, changes in the activity and expression of enzymes of glucose and lipid metabolism, and modifications in the expression level of key transcription factors. We discuss here the emerging view that β-cell “glucotoxicity” is in part indirectly caused by “lipotoxicity,” and that β-cell abnormalities will become particularly apparent when both glucose and circulating fatty acids are high. We support the concept that elevated glucose and fatty acids synergize in causing toxicity in islets and other organs, a process that may be instrumental in the pleiotropic defects associated with the metabolic syndrome and type 1 and type 2 diabetes. The mechanisms by which hyperglycemia and hyperlipidemia alter insulin secretion are discussed and a model of β-cell “glucolipotoxicity” that implicates alterations in β-cell malonyl-CoA concentrations; peroxisome proliferator-activated receptor-α and -γ and sterol regulatory element binding protein-1c expression; and lipid partitioning is proposed.