Fertilization is one of the fundamental processes of living systems. Here I will address marine external fertilization and comment on recent work on mammals. I will show experiments that substantiate that sea urchin sperms exhibit chemotaxis as they swim towards the ovum. They are guided by flagellum internal [Ca2+] concentration fluctuations triggered by the binding of chemicals from the oocyte surroundings. Based on experiment, I present a family of logical regulatory networks for the [Ca2+] fluctuation signaling-pathway that reproduce previously observed electrophysiological behaviors and provide predictions, which have been confirmed with new experiments. These studies give insight on the operation of drugs that control sperm navigation. In this systems biology approach, global properties of the [Ca2+] discrete regulatory network dynamics such as: stability, redundancy, degeneracy, chaoticity and criticality can be determined. Our models operate near a critical dynamical regime, where robustness and evolvablity coexist. This regime is preserved under a class of strong perturbations. Based on global dynamics considerations, we have implemented a network node-reduction method. The coincidence of this reduced network with our bottom-up step-by-step, continuous differential equation modeling is reassuring. For the case of mammals our research has centered on the understanding of capacitation and acrosomal reaction. The first is a process by means of which roughly one third of the spermatozoa acquire the “capacity” to fertilize; the second enables the spermatozoa to penetrate the egg´s surrounding zona pellucida. Overall, our studies might contribute to fertility issues such as the development of male contraception treatments, which is an area of intense research.