Publisher Summary This chapter focuses on the cestode gametogenesis and embryonic development up to the formation of the oncosphere. A common basic pattern of embryogenesis in pseudophyllideans and cyclophyllideans representing the main groups in Cestoda is discussed. Embryonic development in Cestoda shows some specificity in comparison with other animals. Three phases of embryonic development can be distinguished: cleavage, the preoncospheral phase, and the oncospheral phase. In spite of their peculiarities, these phases are comparable to the phases of embryonic development recognized in other animals. Cleavage is the period of early segmentation of the ovum. The preoncospheral phase is the one following cleavage and leading to the formation of the oncosphere. It can be regarded as a phase of morphogenesis. The preoncospheral phase is one of intense cell multiplication and differentiation and it is a phase of great metabolic activity in the embryo. Recognition of these phases has some value in respect of physiological differences, and it is useful in comparative studies of embryos. A common general pattern of embryogenesis in cestodes is summarized in the chapter.