The present invention generally relates to non-linear imaging systems and methods for use in various assisted reproductive technologies and other applications. In one aspect, the present invention is generally directed to two-photon and other non-linear microscope techniques that can be used to determine the status or maturity state of eggs or egg follicles, for example by determining the presence of polar bodies within the egg follicles. In some embodiments, endogenous metabolites, such as NADH or FAD, can be determined, and in some cases without externally altering those metabolite concentrations or without adding other compounds. Other embodiments of the invention are also directed to computer programs or systems that can be used to determine the status or maturity state of eggs or egg follicles, kits in such systems, or the like.