This document outlines the key concepts of the Market Systems Development (M4P) approach. It discusses how markets often fail to serve the poor due to limited access to information, weak institutions, and high transaction costs. The M4P approach aims to facilitate sustainable change in the market system through a light-touch and flexible role. Interventions identify leverage points and partners to stimulate systemic changes, such as encouraging new relationships between private sector actors, that ultimately benefit large numbers of poor people through improved access to markets.