Sexually reproducing land plants in general strictly obey the one seed-one embryo rule, whereby a single mature embryo originates from a single fertilized egg cell. Polyembryony in seeds plant is the development of more than one embryo per seed, and can be classified into so-called multiple and simple forms of polyembryony depending on the origin of the additional embryo. Although polyembryony represents a characteristic byproduct of apomixis, various forms of sexual (zygotic) polyembryony have been observed. However, the knowledge on these forms of polyembryony remains scarce, particularly in terms of identifying causal genes and elucidating molecular mechanisms. Here we provide a comprehensive overview of current knowledge on polyembryony, encompassing classification, mutants, genes, and we address the underlying mechanisms from the research performed in angiosperms.