Dear colleagues, We are pleased to announce the publication of a special issue of Marine Mammal Science that defines subspecies and then focuses on using molecular genetic data to delimit cetacean subspecies. The editors introduce marine mammal subspecies in the age of genetics. The papers in the special issue then discuss: 1) the need to advance taxonomy for cetaceans, appropriate definitions for *populations, subspecies and species* and the rationale for relying largely on genetic data; 2) review recent studies that have used genetic data to delimit cetacean subspecies and species; 3) review analytical methods used for such studies; 4) evaluate the performance of different commonly-used metrics for subspecies delimitation; 5) present a new application of the Random Forests method to subspecies delimitation; and 6) provide guidelines and quantitative criteria for use in future studies of cetacean subspecies. The guidelines and criteria are not intended to be applied rigidly but should promote consistent taxonomic arguments. The authors hope the set will evolve with the oversight of the Society for Marine Mammalogy taxonomy committee. A list of the papers that appear in the special issue is below. All of the papers are available online at http://onlinelibrary.wiley.com/doi/10.1111/mms.2017.33.is sue-S1/issuetoc Cheers, Karen Martien, on behalf of all co-authors Papers in special issue: Schwartz, M.K. and D.J. Boness. Marine mammal subspecies in the age of genetics: introductory remarks from the Associate Editor and Editor-in-Chief of *Marine Mammal Science*. Taylor, B.L., W.F. Perrin, R.R. Reeves, P.E. Rosel, J.Y. Wang, F. Cipriano, C.S. Baker, and R.L. Brownell, Jr. Why we should develop guidelines and quantitative standards for using genetic data to delimit subspecies for data-poor organisms like cetaceans. Martien, K.K., M.S. Leslie, B.L. Taylor, P.A. Morin, F.I. Archer, B.L. Hancock-Hanser, P.E. Rosel, N.L. Volymer, A. Viricel, and F. Cipriano. Analytical approaches to subspecies delimitation with genetic data. Rosel, P.E., B.L. Taylor, B.L. Hancock-Hanser, P.A. Morin, F.I. Archer, A.R. Lang, S.L. Mesnick, V.L. Pease, W.F. Perrin, K.M. Robertson, M.S. Leslie, A. Berta, F. Cipriano, K.M. Parsons, A. Viricel, N.L. Vollmer, and K.K. Martien. A review of molecular genetic markers and analytical approaches that have been used for delimiting marine mammal subspecies and species. Rosel, P.E., B.L. Hancock-Hanser, F.I. Archer, K.M. Robertson, K.K. Martien, M.S. Leslie, A. Berta, F. Cipriano, A. Viricel, K.A. Viaud-Martinez, and B.L. Taylor. Examining metrics and magnitudes of molecular genetic differentiation used to delimit cetacean subspecies based on mitochondrial DNA control region sequences. Archer, F.I., K.K. Martien, and B.L. Taylor. Diagnosability of mtDNA with Random Forests: using sequence data to delimit subspecies. Taylor, B.L., F.I. Archer, K.K. Martien, P.E. Rosel, B.L. Hancock-Hanser, A.R. Lang, M.S. Leslie, S.L. Mesnick, P.A. Morin, V.L. Pease, W.F. Perrin, K.M. Robertson, K.M. Parsons, A. Viricel, N.L. Vollmer, F. Cipriano, R.R. Reeves, M. Krützen, and C.S. Baker. Guidelines and quantitative standards to improve consistency in cetacean subspecies and species delimitation relying on molecular genetic data. -- Karen K. Martien, Ph.D. Southwest Fisheries Science Center 8901 La Jolla Shores Dr. La Jolla, CA 92037, USA Phone: 858-546-7058 Fax: 858-546-7003 Karen.Martien@noaa.gov http://swfsc.noaa.gov/mmtd-mmgenetics