Maternal effects, which include the contribution of the maternal parent to the offspring beyond the equal chromosome contribution of each parent, may be produced by different mechanisms. Maternal effects may arise through cytoplasmic factors in the transmission of organelles, by endosperm effects, and by phenotypic effects, which include the influence of both the maternal genotype and the maternal environment. The chapter focuses only on the maternal effects to maternal environmental effects. A maternal trait that has often been studied is seed mass. Seed mass may affect different juvenile and adult characters and is probably one of the earliest indicators of offspring quality. Since a large part of the seed tissues are of maternal origin, it is not surprising that the maternal parent has a large effect on seed mass. Maternal effects may influence a variety of traits, particularly those related to fitness, and thus may be ecologically and evolutionarily important.