This review on biguanides gives a general survey of biguanide actions on many metabolic models in vivo and in vitro. It focuses especially on the comparison of the 3 antidiabetic biguanides, of which metformin is discussed as the only oral antidiabetic biguanide useful in treatment of type-II diabetes with a minimum risk of side effects when applied in a well controlled regime. Pharmacokinetics, pharmacodynamics, hypotheses on the molecular mode of action, and implications for therapy are discussed.