Experience and knowledge gained by the activities of an ODA Plant Pathology Project are used to review aspects of plant protection in Northern Thailand, including important plant diseases, availability and development of extension services, pesticide use and the introduction of pests and diseases on exotic crops. Full records of fungal and bacterial diseases on 100 host plants are provided as an annotated list. Special attention is paid throughout to highland development programmes aimed at finding suitable subsistence and cash crops for the hill tribes to replace opium.