This document discusses the dynamic and complex nature of agri-food systems and argues that the prevailing approaches to agricultural science and policy often fail to provide sustainable outcomes, especially for poor people in developing countries. It outlines two perspectives in agricultural science - a holistic, systems-based approach versus an orthodox, equilibrium-focused approach. A holistic approach that considers uncertainty, diversity and complexity is needed to better understand agri-food systems and define practices and policies that can help systems become more resilient to shocks and stresses. The document examines drivers of change in global agri-food systems and characteristics of diverse rural livelihoods to provide context for later discussions of sustainability narratives and pathways.