One of the key questions of primary importance to global agriculture and food security is how to optimize sustainable intensification to balance competing demands on land for food and energy production, while ensuring the provision of ecosystem services and maintaining or increasing yields. Integrating trees and agriculture through agroforestry has been attracting increasing interest as an agroecological approach to sustainable intensification. Trees have traditionally been important elements of temperate agricultural systems around the world, but there has been increasing separation of agriculture, forestry and nature over the past few decades. This paper discusses what we can learn from traditional agroforestry systems to help develop modern systems that integrate ecological farming and agroecological advances to achieve sustainable intensification. We also discuss the existing barriers to wider adoption of agroforestry, and identify how these barriers can be overcome to promote agroforestry as a mainstream land-use system.