Based on extensive new material from North American Pacific coastal marine waters, and on the previous literature, the nominate subfamily, Corophiinae, is newly proposed within family Corophiidae Dana, 1849, to encompass the polymorphic genus Corophium Latreille, 1806 sens. lat. World-wide, the new subfamily embraces 13 genera world-wide of which 6 relatively primitive genera have unfused urosomal segments and member species are primarily free-burrowing in soft sediments, and 7 relatively advanced genera that have fused urosomal segments and component species typically construct open-ended tubes on hard substrata. The following 10 genera are represented in the North Pacific region: Eocorophium, new genus [Type species - E. kitamori (Nagata, 1965)]; Sinocorophium, new genus [Type species - S. sinensis (Zhang, 1974)]; Americorophium, new genus [Type species - A. spinicorne (Stimpson, 1857)]; Microcorophium, new genus [Type species - M. miospinulosum (Hirayama, 1986)]; Lobatocorophium, new genus [Type species -L. lobatum (Hirayama, 1987)]; Crassicorophium, new genus [Type species - C. crassicornis (Bruzelius, 1859)]; Hirayamaia, new genus [Type species -H. mortoni (Hirayama, 1986)]; Monocorophium, new genus [Type species - M. insidiosum (Crawf (Chevreux, 1908)]; and Laticorophium, new genus [Type species - L. baconi (Shoemaker, 1934a)]. The following 3 genera are restricted variously to the northeastern North Atlantic, Mediterranean and Aralo-Caspian regions: Corophium Latreille, 1908 [Type species - C. volutator (Pallas, 1776)]; Medicorophium, new genus [Type species - M. aculeatum (Chevreux, 1908)], and Chelicorophium, new genus [Type species - C. chelicorne Sars, 1895b)]. In the present study of the subfamily in coastal marine and brackish waters, from Alaska to northern California, the following species are recorded, redescribed, variously refigured, and keyed. Americorophium spinicorne (Stimp.),A. salmonis (Stimp.), A. stimpsoni (Shoem.),A. brevis (Shoem.), Crassicorophium crassicorne (Bruz.), Monocorophium insidiosum(Crawf.),M. steinegeri (Gurj.), M. acherusicum (Costa), M. californianum (Shoem.), and Laticorophium baconi (Shoem.). Newly described within Monocorophium and recorded here is M. carlottensis, new species. Five other regionally peripheral species, rassicorophium clarencense (Shoem.) and C. bonelli (M.-E.), arctic and subarctic shallow-water species, Monocorophium oaklandense (Shoem.), a cool water Californian endemic, and M. uenoi (Nagata) and Sinocorophium alienense (Chapman), synanthropic in San Francisco Bay, are also redescribed, figured and keyed. Taxonomically, previously employed character states such as the degree of fusion of the urosomal segments, form of the mandibular palp, and degree of ventral insertion of uropods were found to exhibit homoplasious similarity in otherwise distantly related genera, and thus appear unreliable as a basis for cladistic classification within the subfamily. Phyletically, the endemic North American Pacific corophiin fauna exhibits strong affinities to that of the western North Atlantic, but little to that of the eastern North Atlantic and Mediterranean regions, and apparently none to the Asiatic North Pacific fauna. Of the warm-temperate species of Monocorophium from central and southern California, only Corophium californianum ranges northward to British Columbia. The three species of Crassicorophium occurring in northern parts of the study region are arctic-subarctic in biogeographic affinities and may be closest phyletically to the primitive, mainly sublittoral Mediterranean endemic genus Medicorophium. We may conclude that the Corophiinae are relatively recently evolved, and have two main centres of origin.