Technical Abstract: Until recently, the cellular and molecular events comprising fertilization in birds were rather poorly understood. However, several recent studies in mammals as well as in birds have considerably contributed to our knowledge of the fertilization process. Mammals exhibit physiological monospermy, that is, only a single sperm enters the ovum. Monospermy is ensured by a variety of mechanisms, which is collectively known as the block to polyspermy. Unlike mammals, the principal feature of fertilization in birds, as well as in many fishes, amphibians and reptiles, is physiological polyspermy, that is, penetration of the ovum by many sperm. It has been suggested that polyspermy occurs in animals that produce large yolky (megalecithal) ova. This review is not a comprehensive account of all aspects of avian fertilization, but rather concentrates on the following: oviductal sperm selection, storage and transport; the interaction of gametes; the role of ovum¿s envelope; and formation and further fate of pronuclei. The main focus is on the complexities of polyspermic fertilization and the mechanisms preventing excessive pathological polyspermy that ensure normal embryonic development. This chapter also includes some data on assisted reproductive technologies used in birds, such as artificial insemination (AI), in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI). Although the latter two methods have yet to find practical application, they can shed some light on at least some mechanisms of avian fertilization in vivo. Most of the available information on avian fertilization relates to the chicken (Gallus gallus), since it is commonly used as an experimental bird. In this chapter, all observations will be derived from research with the chicken unless otherwise noted.