The modern Sirenia have been little studied because of their present rarity and the difficulty of observing fully aquatic animals even though so large. These mammals are the only large aquatic herbivores. One of the three modern forms, Steller's seacow, is recently extinct; the dugong of the tropical Indo-Pacific region is now rare almost everywhere apart from some of the northern waters of Australia, while the manatee of the two sides of the tropical Atlantic is both marine and riverine but has become very scarce. The two authors have, over 12 years, endeavoured by means of personal visits to the Guianas, Australia, Torres Strait, Ceylon and elsewhere, and by copious correspondence, to gather material so as to summarize the present distribution and status of the Sirenia. This information is presented in a setting of conservation and eventual rational use as meat animals and in other ways. There is also reference to the history and manner of exploitation which has brought the Sirenia to their present lowly state. The main residual stock is that in northernmost Australian waters, and there numbers may now indeed begin again to increase.