Seed dimorphism (polymorphism) can be defined as the production of two (or more than two) different types of seeds by a single individual. The seeds may be different in morphological and/or physiological aspects. Seed dimorphism or polymorphism is found in many plant species of Asteraceae, Poaceae, Brassicaceae and Chenopodiaceae. Dimorphic seeds typically differ in the presence/absence of dispersal structures (e.g., pappus, trichomes). In members of Asteraceae, peripheral achenes are persistent, large, dormant and sensitive to light, temperature, water and salt; while central archenes are dispersible, small and non-dormant. Dormant seeds or the seeds which are sensitive to environmental factors are an essential part of a seed bank. Seedlings from dimorphic seeds differ at the early stage (40 days old for Hedypnois cretica), but the difference decreases quickly for H. cretica(no difference at 70 days old), while the difference persists during the life cycle for Atriplex Sagittata. Both kinds seedlings produce dimorphic seeds in next generation. Environmental factors such as water, salt, nutrient and density stress affect the ratio of dimorphic seeds. Genetic factors also affect this ratio with a heritability factor of 0.2～0.5. Seed dimorphism is a quantitative genetic character and is under the control of more than two genes (locus). In order to explain the causes of seed dimorphism, two models are proposed based ecological and ontogenetic constraints: bet-hedging (high risk/low risk) strategy and ontogenetic model. Dimorphic seeds from different individuals of the same species differ considerably in their structure, germination/dormancy characters, responses to environmental stresses, early seedling growth and genetic aspects. Generally, seed dimorphism (polymorphism) is a consequence of long-term (evolutionary) interactions between environmental and genetic factors - an ecological adaptation to variable environment. The underlying mechanisms of seed dimorphism are very complex and unclear. Four guidelines are proposed for future studies: biotechnology procedures to identify genetic mechanisms of seed dimorphism, molecular biology techniques to identify phytohormone differences in dimorphic seeds, the current ecophysiological studies to understand the relative importance of various environmental factors and life cycle research.