The formation of ovules is a key step in the plant life cycle which alternates between a diploid and haploid generation, the sporophyte and the gametophyte. The transitions between the two generations in the female occur in the ovule, the site of meiosis, female gametogenesis and double fertilization. The intimate association of sporophytic and gametophytic tissues in the ovule allows an investigation of their cellular interactions during ovule and seed development. In Arabidopsis, several sporophytically acting loci which primarily affect the morphogenesis of the integuments have been identified. Early acting genes may respond to positional information leading to a region-specific initiation of morphogenesis, whereas later acting genes affect cellular aspects of integument growth. Much less is known about the mechanisms controlling the development of the megagametophyte. Comparative morphology and genetic analyses suggests that regulatory mechanisms ensure a tight coordination of independently controlled cellular processes with cell specification and differentiation.