Bats and dolphins echolocate ultrasonically while foraging, an active mode of perception that is effective for intercepting small, fast-moving targets, but less so for tracking large targets from long distances. Unlike toothed whales, humpback whales and other baleen whales are widely assumed not to echolocate. Echoes generated by humpback whale vocalizations often have been viewed either as low-level background noise that minimally affects acoustic communication between whales or as indirect cues to large environmental features. An additional possibility is that vocalizing humpback whales (and perhaps other whales) produce sonic sounds to actively generate echoes that they localize and interpret. Animals attempting to echolocate using lower frequency sounds underwater face some of the same signal processing challenges faced by ultrasonic echolocators, as well as some unique challenges. The extent to which listening whales can meet these challenges largely determines what they can actively perceive by monitoring self-generated echoes. In the absence of psychophysical experiments with baleen whales, comparisons with auditory processing by echolocating species provide the best indications of what humpback whales may echoically perceive. Empirical tests of key predictions derived from the hypothesis that humpback whales echolocate are also critical to understanding how they use sound. Recognizing that sonic echolocation by whales may differ substantially from ultrasonic echolocation in smaller species is an important step toward clarifying the role that active auditory perception plays in whale behavior. (PsycINFO Database Record (c) 2020 APA, all rights reserved).