The marine soundscape is made up of natural ambient sounds (e.g. wind and waves), biological sounds (e.g. animal calls) and anthropogenic sounds (e.g. ship noise). Acoustic ecology studies the relationships— mediated through sound—between organisms and their environment. As ocean water conducts light very poorly yet sound very well, marine mammals rely heavily on acoustics for communication and navigation. Since the onset of the industrial revolution, man-made noise in the ocean has steadily increased. The effects of noise on marine animals can be short-term or long-term, transient or chronic, negligible to biologically significant, where the survival of a population is at risk. This article gives an overview of the components constituting the marine soundscape, of the use of sound by marine mammals and of the effects of noise. The acoustic ecology of animals other than mammals and the effects of noise on animals other than mammals are less understood.