Technical Abstract: Plant defence theory provides a robust framework for understanding interactions between plants and antagonists, and for interpreting broad patterns in the functional-trait composition of plant communities. However, this framework has been built almost entirely on traits expressed by seedlings and mature plants. No equivalent seed defence theory exists that recognises the distinct suite of natural enemies that seeds encounter, and the unique opportunities and constraints to their response. Furthermore, most attention has been paid to insect and vertebrate seed predators active above ground, whereas microbes in soil also have large effects on seed survival, particularly for plants that recruit following infrequent disturbances and rely on germination from soil seed banks. We suggest that selection should result in seed defence syndromes, analogous to the defence syndromes expressed by plants after germination. Syndromes should arise from concurrent selection on seed dormancy and resistance to microbial decay: physiological traits of seeds are related to the timing and predictability of recruitment opportunities, resulting in a range of seed dormancy states that confer varying degrees of protection from decay by soil pathogens. We predict that seeds with physical dormancy will rely on physical seed defences to exclude predators and pathogens, and rapid seed germination to escape pathogens at the emergence stage. In contrast, seeds with transient seed banks and those with physiological dormancy will deploy a continuum of physical and chemical defences, depending upon the level of soil pathogen pressure. Finally in some ecosystems, seeds persist in the soil in a non-dormant, imbibed state, and lack obvious chemical and physical defences. These seeds may be especially dependent upon protection against pathogens by seed-inhabiting microbes.