This work employed the ‘integrated co-learning approach’ which we developed and tested in western Kenya. The aim of the approach was to facilitate sustainable intensification and thereby to increase crop yields and household income of smallholder farmers. During the five seasons of the programme most households became maize self-sufficient as yields doubled or tripled. Provision of input vouchers each season allowed farmers to invest in technology such as seeds, fertilizer and other inputs. Yet due to small farm sizes most households were not able to earn a ‘living income’ despite the large increases in yields. Co-learning proved key to facilitate the complex changes in the cropping system that are required for sustainable intensification, such as the incorporation of legumes. Building on the finding that farm size limited farmer income, we developed a novel benchmark of the ‘viable farm size’: the cultivated area required to attain a living income from arable farming. With current yields, farm sizes would have to be 1.3-4.5 times larger to attain a living income. By contrast, most households could attain a living income with their current farm size if they could increase their crop yields. This thesis underscored the need for large changes within and outside the farming sector to sustainably increase yields and farmer incomes, while ensuring no one is left behind.