Fertilization by intracytoplasmic sperm injection (ICSI) is a powerful technique that can be used to understand better the biology of fertilization, in addition to a form of assisted reproduction both in humans and in endangered species. Mouse is often the model organism of choice to study mammalian fertilization. The ability to fertilize successfully mouse eggs by sperm injection, however, has been hard to achieve. The introduction of piezo-actuated injection made possible to perform mouse ICSI in an efficient fashion. Piezo-actuated ICSI is also currently used to fertilize eggs of other mammalian species, for injection of earlier spermatogenic cells into mouse eggs, as well as for somatic cell nuclear transfer, and cloning. This chapter provides detailed experimental procedures to perform ICSI in mouse eggs.