A series of marine mammal strandings, lawsuits, legislative hearings and, most recently, a report from the U.S. Commission on Ocean Policy, have all drawn attention to the subject of how ocean noise affects marine mammals. One way to assess the impact of ocean noise is to consider whether it causes changes in animal behaviour that are ‘biologically significant’, that is, those that affect an animal’s ability to grow, survive, and reproduce. Marine Mammal Populations and Ocean Noise offers a conceptual model designed to clarify which marine mammal behaviours are biologically significant for conservation purposes.