Cockroaches are among the most common insects. Based on fossil evidence, roaches are known to have been present on Earth for over 300 million years. Their sizes vary considerably; some species are up to several inches long. Some biologists consider insects to be one of the most successful groups of animals to ever inhabit this planet, and cockroaches are one of the most adaptable and successful insect groups. They have been able to survive many changing environments over millions of years. There are approximately 3500 species of cockroaches worldwide-about 60 species are found in the United States. Their presence in nearly every part of the world and wide range of habitats demonstrate that cockroaches are truly an outstanding success story in Nature. Unfortunately, many of the same outstanding biological characteristics which make them so successful, also make them one of the most difficult pests to manage. Cockroaches, especially the German cockroach, are the most commonly encountered and important household insect pests in much of the U.S.