The underwater existence of marine mammals has encouraged a variety of special biophysical adaptations to their environment. Their sensory and communication systems reflect the transmission properties of sea water. For example, vision is keen in spectra that penetrate water best, vocalization is broadband and used at the frequencies that appear to fit their activities best—the differences in sensory use match the intriguing variety of behavior observed for each species. To date most of the observations of animal interactions with their marine environment have dealt with sound. There has been some work on vision and studies are underway to determine animal sensitivities to hydrodynamic pressure, chemical traces and magnetic fields. The species that have been recorded to date are listed and vocalizations are generally compared. Methods for observation of sensory mechanisms are noted along with a discussion of other aspects of marine mammal biophysics including vibrissal sensation and the biophysics of movement in a fluid environment.