One of the little known species in the danger of extinction, gymophionan amphibians, are also known as caecilians or apoda. Gymnophiona contains 154 species belonging to 34 genera and 6 families. For many years, studies on the Gymnophiona were disparate and still only a few species have been deeply studied. Fortunately, in recent years, some new works have been published on their systematics, using both the classical methods as well as immunology and molecular biology. New data have also been obtained on the biology, life history, reproductive biology, endocrinology and embryonic development of several species. These fascinating aspects along with other important ones on gymnophionan studies are ably reviewed in this book.