This is the final version of a survey on many-valued logic to appear as Chapter 2 of Volume 1 "Quantified Representation of Uncertainty and Imprecision" of a "Handbook of Defensible Reasoning and Uncertainty Management Systems" to be published by Kluwer. The editors of the Handbook are D. Gabbay and P. Smets, and the editors of Volume 1 are D. Gillies and P. Smets. Contents 4 Why many-valued logic? 30 References 31