This book provides an overview of our current understanding of polyembryony in insects. The study of polyembronic insects has advanced considerably over the last several decades. The book shows the exciting potential of polyembryonic insects and their impact on life sciences. It describes the mechanisms of polyembryogenesis; tissue-compatible invasion of the host, which is the first case of compatible cellular interaction between phylogenetically distant organisms without rejection; the sex differences in defense; and the environmental regulation of caste structure.  The first book devoted to polyembryony in insects, it draws on the author research on polyembryonic wasps from 1990 to the present day, covering various topics such as polyembryogenesis in vitro, host-parasite interaction, sex differences in soldier function/humoral toxic factor, and the transcription analysis of polyembryogenesis. It is intended not only for researchers in the field of entomology, parasitology, ontogeny, reproductive biology, developmental biology, sociobiology, and evolutionary developmental biology (Evo-Devo), but also for postgraduate students in these fields.