In an era when health resources are increasingly constrained, international organisations are transitioning from directly managing health services to providing technical assistance (TA) to in-country owners of public health programmes. We define TA as: 'A dynamic, capacity-building process for designing or improving the quality, effectiveness, and efficiency of specific programmes, research, services, products, or systems'. TA can build sustainable capacities, strengthen health systems and support country ownership. However, our assessment of published evaluations found limited evidence for its effectiveness. We summarise socio-behavioural theories relevant to TA, review published evaluations and describe skills required for TA providers. We explore challenges to providing TA including cost effectiveness, knowledge management and sustaining TA systems. Lastly, we outline recommendations for structuring global TA systems. Considering its important role in global health, more rigorous evaluations of TA efforts should be given high priority.