Agricultural mechanization has brought far reaching changes in farming structure in all developed countries, and its effects are now beginning to be felt in many developing countries. Within the former, mechanization has been associated with increasing farm size, migration of labour out of farming, and the development of agriculture as a specialized commercial activity. New inputs of improved quality have been developed, modern management practices adopted, and machinery inputs (capital) have substantially substituted for labour and animal power inputs. The impact of farm mechanization results from its interaction with the institutional structure of agriculture, however, and introduction to the developing countries will not necessarily result in similar adjustments. Indeed, one might argue that it is the duty of the developing countries to intervene and modify the impact of mechanization. Before such intervention, however, it is necessary to understand the effects of mechanization on factors such as output, labour demand, and cropping intensity. Equity considerations require an assessment of the impact on farm earnings, as well as the income effect~ on landless labourers. The problem is one of disentangling cause from effect and distinguishing between the factors which cause change and those which merely provide the means. One view of mechanization holds that it is directly output increasing; another that it has little effect. This is an empirical issue and results are likely to be site specific. Farm structures, land:labour ratios, tenure patterns, soils, prices, irrigation networks, and institutional arrangements vary from site to site, and will both shift the underlying production function and shape the behaviour of farmers.