Acoustic censusing of marine mammals is an advancing technique. This presentation will discuss advantages and disadvantages of acoustic censusing compared to visual censusing. When considering using acoustics to describe marine mammal populations, several questions need to be addressed, including type of population estimate (relative or absolute), array design, localization requirements, frequency bandwidth, and species diversity of the sampled population. Major advantages of acoustic censusing include greater detection ranges, fewer environmental limitations, and a complete record of all contact cues. Given recordings of each contact, the signals can be further analyzed for source identification and localization. Acoustic censusing difficulties relate to determining source identity, group size, and detection distance. Fundamental choices such as array design can have major impacts. Finally, the overall advantages of acoustic censusing, particularly when done concurrently with a visual survey lead to useful data. In the recently completed GulfCet I project, acoustic effort occurred along 95% of the survey track, compared to 49% for the concurrent visual survey. Population estimates from the acoustic survey were 316 (265-377) sperm whales and 36,946 (33,512-40,566) dolphins, compared to 313 (192-508) sperm whale and 18,584, (10,268-35,431) dolphins for the visual survey. Subsequent analysis can now be done, for example, on the effects of noise on marine mammals based on signals recorded during the survey.