Maize seeds, like those of all other angiosperms, are highly complex biological systems. This complexity is a consequence of the fact that the angiosperm seed is composed of tissues that evolved from three genetically distinct organisms: the mother plant (maternal sporophyte-specifically the nucellus, integuments, and in the case of maize and other cereals, other floral organs that fuse with the integuments to form the pericarp); the developing embryo (zygotic sporophyte); and the endosperm (arising through fertilization-dependent proliferation of a second fertilization competent cell of the female gametophyte). In evolutionary terms, the two main interactions that will be addressed in this chapter initially arose well before emergence of the angiosperms. The first and probably most ancient association is "retention and nourishment" of the sporophytic embryo by the female gametophyte. The second association is derived from the "retention of the megaspore" by the maternal sporophyte, and the subsequent extended interaction of maternal tissues (the nucellus and integuments) with the developing female gametophyte (and subsequently, in angiosperms, the endosperm). This chapter addresses interactions that occur at compartment interfaces. After a short description of their structures, it focuses mainly on three aspects: (i) the potential role of signalling peptides (and other mobile substances) and their receptors as signalling components in cell fate decisions and intercompartmental communication; (ii) the role of sugars both as nutrients and as signalling molecules; and (iii) the role and control of cell death processes as a consequence of physical interactions and as a potential source of signalling molecules for intercompartmental communication.