Forests and tree-based systems are an important component of rural landscapes, sustaining livelihoods and contributing to the food security and nutritional needs of hundreds of millions of people worldwide. Historically, these systems developed under a wide variety of ecological conditions, and cultural and socio-economic contexts, as integrated approaches that combined management of forest and agricultural areas to provide primarily for the needs of producers and their local communities. Today they serve food and nutrition demands of growing global populations, both urban and rural. Population increase, globalisation, deforestation, land degradation, and ever-increasing demand and associated conflict for land (including forest) resources are placing pressure on these lands. Farmers have been encouraged to intensify food production on existing agricultural lands, by modifying some traditional practices (such as agroforestry) or abandoning others (such as shifting cultivation) that evolved over centuries to cope with biophysical constraints (e.g. limited soil fertility, climate variability) and changing socio-economic conditions. This chapter provides an overview of forests and tree-based systems and their role in enhancing food security and nutrition for rural communities and those served through the marketplace. The variability and viability of these management systems are considered within and across geographical regions and agro-ecological zones. Also discussed is the role of the social, cultural and economic contexts in which these systems exist, with a focus on three factors that affect the socio-economic organisation of forests and tree-based systems, namely: land and tree tenure and governance, human capital (including knowledge and labour) and financial capital (including credit). How these biophysical and socio-economic conditions and their complex interactions influence food security and nutrition outcomes, particularly for vulnerable segments of the population (i.e., the poor, women and children), are of particular concern.