Feral pigs (Sus scrofa) are an exotic ungulate which have been widely introduced worldwide with multiple ecosystem and economic consequences. The author conducted a semi-comprehensive literature review directed at identifying the current state of knowledge related to the effects of feral pigs on island and mainland plant and animal communities. Also, the author describes the situation in California where feral pigs that were introduced in the late 1700s are now widespread due to hunting-related introductions and natural range extensions. Feral pigs on predator-free oceanic islands are a serious conservation problem because they attain high densities and have contributed to near-extinctions and extinctions of multiple endemic plants and vertebrates. In mainland ecosystems, however, feral pigs can have both positive and negative effects depending on the local circumstances. Rooting, for example, can have both positive and negative effects on growth and survival of some trees, soils and soil processes, and the distribution of native and exotic grasses. In general, however, the negative effects of rooting by feral pigs are amplified when population densities are high. Feral pigs may compete with native species for limited resources, but there are limited data relevant to this hypothesis. Based on observations of small amounts of animal matter in their diets, feral pigs eat terrestrial vertebrates and eggs of ground nesting birds, but the importance of predation by feral pigs on native vertebrates is poorly known. Feral pigs also may have important indirect effects in mainland ecosystems by providing a new prey base for native predators which may then increase. In areas of Europe with extant wolf (Canis lupus) populations, wild boar (Sus scrofa) are an important prey species which may be facilitating numerical and geographic recoveries of wolves. Because wild boar are important prey for endangered Amur tigers (Panthera tigris), they are considered important for recovering tiger populations. In Australia, feral pigs are potentially important prey for dingoes (Canis familiaris dingo); whereas, in the United States, endangered Florida panthers (Felis concolor coryi) consumed 23% to 59% feral pigs, and mountain lions (Felis concolor) in Texas and California consumed 5% to 38% feral pigs. Research needs for feral pigs include quantitatively assessing: 1) how acorn foraging by feral pigs limits or influences regeneration of oaks (Quercus sp.); 2) the competitive effects of feral pigs on native species; 3) whether direct predation by feral pigs suppresses small vertebrate populations; and 4) how the availability of feral pigs as prey influences native predator populations.