Seed transmission is the method par excellence by which plant pathogens are introduced into new areas, survive periods when the host is absent, are selected and spread as host-specific strains, and are disseminated as random infection foci in plant populations. Pathogens may accompany seed on debris, or may be carried externally or internally with the seed. The successive transfers of the pathogen (plant to seed, seed to seed, seed to plant and plant to plant) are complex and strongly affected by the physical environment, the seed and soil microflora and the type of seed germination. Seed transmission may be diminished or prevented by selecting production areas unfavourable for disease development, culture practices, harvesting and cleaning procedures, seed field inspection, seed indexing, and chemical and thermotherapy seed treatments. Twenty two diseases of flower crops are discussed in relation to these factors of seed transmission.