Seeds are the organs by which higher plants disperse and propagate. Seeds are relatively insensitive to stress-inducing environmental conditions and can survive in the soil for many years in a dormant state. The alleviation of dormancy results in germination and finally seedling growth and these processes are complex and subject to hormonal control. Many plant species produce ethylene during the final phase of seed development and germination, and the application of some seed dormancy-breaking agents, for example HCN or NO, also induces ethylene production. Therefore, it has been proposed that ethylene contributes to the alleviation of seed dormancy as well as germination. The action of ethylene during dormancy release and germination also depends on interactions with other phytohormones, and so this chapter summarizes our current knowledge on the role of ethylene as well as crosstalk between ethylene and the other phytohormones (gibberellins, abscisic acid, brassinosteroids, jasmonate and polyamines). Further, other signalling molecules such as reactive oxygen species (e.g. H₂O₂) or reactive nitrogen species (e.g. NO) and HCN are also discussed in terms of their roles in the regulation of seed physiology.