Accidental introductions into the marine environment have been either by hull-fouling or ballast water. While a large number of species has been introduced accidently (more than 150 species are known), only a few are classified as pests. Four selected for discussion are the Seastar Asterias amurensis, the Crab Carcinus maenas, the Polychaete Worm Sabella spallazanii and the Bivalve Musculista senhousia, these, together with toxic dinoflagellates, show how introduced species may impact on the marine environment. All four species appear to be expanding their range. In most cases, little is known about them and most attention has been devoted to documenting their distribution rather than studying their impact. Many other introduced species may also be pests, but our knowledge of coastal systems is such that they are not currently recognized. Finally, a plea is made t o implement control mechanisms to prevent the expansion along our coasts of species designated as pests. Studies also need to be implemented to assess the impact of these pest species on the marine ecosystem, rather than relying on anecdotal information. Species not currently recognized as pests may also need to be included in these studies.