The California Channel Islands, U.S.A., and Northwestern Baja California Islands, Mexico, host important breeding populations of several seabird species, including the endemic Black-vented Shearwater (Puffinus opisthomelas) and Xantus' Murrelet (Synthliboramphus hypoleucus). Mammals introduced to nearly all of the islands beginning in the late 1800s to early 1900s include: cats (Felis catus), dogs (Canis familiaris), Black Rats (Rattus rattus), rabbits and hares (Leporidae), goats (Capra hirca), sheep (Ovis ones), and other grazers. Cats, dogs and rats are seabird predators, grazers such as goats and sheep cause habitat degredation, and rabbits destroy habitat and compete with hole-nesting seabirds. Cats, which were introduced to at least 19 islands and currently occur on ten islands, have had the greatest impacts on seabirds, including the extinction of the endemic Guadalupe Storm-Petrel (Oceanodroma macrodactyla). Cats are known to have eliminated or severely reduced colonies of Black-vented Shearwaters, Cassin's Auklets (Ptychoramphus aleuticus) and Xantus' Murrelets. Black Rats have occurred on a minimum of seven islands and have reduced numbers of small, hole-nesting alcids on at least one island. At many islands, defoliation and erosion caused by rabbits and large grazing mammals has been severe. Their effects on seabirds are not well documented but potentially are serious. Impacts from introduced mammals have been most severe on islands with no native mammalian predators. On the Northwestern Baja California Islands, temporary and permanent human settlements have led to a greater diversity and source of introductions. Programs to remove introduced mammals and to reduce the possibility of future introductions are needed to restore seabird populations and to preserve the biodiversity of the region. Surveys are needed particularly on the Northwestern Baja California Islands to update the status and distribution of seabirds and to further assess impacts from introduced mammals.