Aquaculture is one of the most rapidly developing sectors of global food production, contributing significantly to socio-economic development by providing food security and export earnings from high-value products. The importance of the industry to the expanding world population is underscored by the growing gap between supply and demand for seafood. However, rapid growth in aquaculture has been accompanied by increasing problems with disease, and disease is now seen as a very significant threat to sustainable production and trade. During the past decade, disease has devastated prawn farming in Asia and Latin America, with annual losses of up to US$3 billion. Devastating losses have also hit the salmon farming industries of Europe and North America, the diverse fish farming industries of Japan, Taiwan and China, and shellfish production in Asia and Europe. Disease in aquaculture is the result of ecological disturbances that disrupt the natural balance between pathogens and their host. Farming is often conducted in unnatural habitats which provide opportunities for exposure to new pathogens. Culture conditions may be potentially stressful, causing existing infections to become more severe and precipitate disease outbreaks. Stocking densities are often high, increasing the risk of disease transmission and spread. There is also a growing trade in live aquatic animals, inevitably resulting in trans-boundary spread of disease and subsequent imposition of trade barriers. Solutions to the growing disease menace lie in improved methods for rapid disease diagnosis and pathogen detection, increased regulation of trans-boundary movement of live aquatic animals, closed-cycle breeding and selection for disease resistance, and improved education and training for both health professionals and farmers.