We describe gross egg morphology and provide the first data on eggshell ultrastructure in cynipoids (Hymenoptera) based on species representing three distinctly different life histories: internal parasitoids of endopterygote larvae, gall inducers and phytophagous inquilines (guests in galls). We then use existing phylogenetic hypotheses to identify putative changes in egg structure associated with evolutionary life-history transitions. We find four major structural changes associated with the shift from parasitoids laying their eggs inside a host larva to gall inducers laying their eggs in or on plants: (1) from a narrow and gradually tapering gross form to a distinct division into a stout body and a long and thin stalk; (2) from a thin to a thick eggshell; (3) from a flexible to a rigid endochorion; and (4) from crystal bundles with shifting orientation in the exochorion to layers of parallel crystal rods. By contrast, we find no major changes in egg structure associated with the transition from gall inducers to inquilines. Comparison between pre- and post-oviposition eggs of one gall inducer and one inquiline suggests that mechanical stress during the passage through the egg canal gives rise to numerous tiny stress fractures in the boundary separating the exo- and endochorion. In one of the gall inducers, Diplolepis rosae, that end of the egg, which is inserted into the plant, has a specialized and apparently porous shell that may permit chemical exchange between the embryo and the plant. Other structures that could facilitate chemical communication with the host plant through the eggshell were, however, not observed in the eggs of gall inhabitants.