This lecture provides an overview of the basic legal principles that underpin the multilateral trade system that existed under the GATT of 1947 and that continue to be an integral part of the WTO. It reviews from an historical perspective, the main market access obligations imposed on WTO Members and the situations in which WTO Members are allowed to set aside those obligations to safeguard their domestic markets or to protect their environment, i.e. when giving priority to policies other than trade.  Finally, it very briefly describes the basic disciplines covering dumping by enterprises and subsidies to ensure undistorted competition.  This overview lecture is the first of a series of more technical presentations on specific sets of WTO disciplines.