Dear colleagues, My co-authors and I are pleased to announce our review chapter on marine mammal reproductive hormones. Orbach, D. N., Sperou, E. S., Guinn, M., & Charapata, P. (2024). Hormones and reproductive cycles in marine mammals. In Norris, D. O., & Lopez, K. H. *Hormones and Reproduction of Vertebrates, *2nd edn, Vol 5, pp. 377-413. Academic Press. https://doi.org/10.1016/B978-0-443-15986-2.00017-4 Abstract: Pinnipeds (seals, sea lions, fur seals, and walruses), cetaceans (whales, dolphins, and porpoises), and sirenians (manatees and dugongs) spend most of their lives beneath the surface of the water. Each clade has distinctive reproductive cycles that more closely resemble their terrestrial relatives than other marine mammal groups, although patterns are relatively conserved within suborders and families. Pinnipeds experience delayed implantation. Postreproductive lifespans are unique to humans and a few species of toothed whales. Male sirenians can restrain, delay, and possibly cease reproductive activity. More is known about the reproductive hormones and cycles of female, captive, and postmortem marine mammals than male, free-ranging, and live animals. However, exploration of diverse tissue matrices and emerging technologies continue to rapidly expand our understandings of marine mammal reproduction. As reproductive cycles are tightly linked to environmental conditions, increasing anthropogenic disturbances pose threats to marine mammal breeding and survival. The article is not open access and is available upon request to Dara Orbach (dnorbach@gmail.com) Thank you, Dara Orbach, PhD Department of Life Sciences, Texas A&M University- Corpus Christi