Attempts to increase fish production and restocking capacities have made synchronous out-of-season spawning necessary. Achieving such spawning is, however, highly dependent on mastering the fundamental control of reproduction during critical reproductive periods in a variety of teleosts. In all teleosts, maturation-inducing hormones (MIHs) are the hormones that mediate the critical stages of final oocyte maturation (FOM) in preparation for ovulation and spawning. Importantly, the identities of MIHs are still unknown or unconfirmed in a number of teleosts, especially those newly introduced into the fish market. Therefore, we here attempt to provide an overview of the most likely MIHs in a wide range of teleosts to aid in confirmation of their identities in already studied species and to suggest their identities in still-unstudied species. In addition to progestogens, various in vitro and/or in vivo studies on a variety of teleosts have yielded other MIHs, such as corticosteroids, prostaglandins, and even melatonin, that are not commonly suspected to be MIHs. Such nonprogestogens can at least interfere with MIH production or function. In all cases, however, both in vitro and in vivo assays should agree on the nomination of a certain MIH before its identity can be confirmed. Altogether, the existing studies strongly suggest that the identity of an MIH can vary among different species, even within the same family of teleosts. Finally, we list the main methods for MIH detection while focusing on in vitro MIH ones considering the paramount importance of such a pursuit in achieving future hormonal treatments for production synchronization in additional teleost species.