The family Limnoriidae is reestablished with keys and diagnoses to all known species including Limnoria (Limnoria) lignorum (Rathke), L. (L.) pfefferi Stebbing, L. (L.) japonica Richardson, L. (L.) septima Barnard, L. (L.) quadripunctata Holthuis, and L. (L.) tripunctata Menzies. The following are described as new species: L. (L.) platycauda, Dutch West Indies, L. (L.) saseboensis, Sasebo, Japan, L. (L.) simulata, Virgin Islands, L. (L.) multipunctata, Kai Islands, L. (L.) unicornis, Ponape Island, L. (L.) foveolata, Kai Islands, L. (L.) sublittorale, N. S. Wales, Australia, L. (L.) insulae, Fiji Islands. Seaweed borers were assigned to the new subgenus Phycolimnoria, containing segnis (Chilton), antarctica (Pfeffer) and the new species algarum, California, segnoides, Misaki, Japan, nonsegnis, Port Arthur, Tasmania, rugosissima, Port Jackson, Australia, and stephenseni, Auckland Islands. The new genus Paralimnoria was instituted to receive the tropical L. andrewsi Calman. Variations in salinity, temperature, dissolved oxygen, and food supply are considered the major factors influencing the distribution of the species. Literature citations are reasonably comprehensive.