This book contains 100 pathways for microbial catabolic metabolism of primarily xenobiotic organic compounds. It includes information on most known microbial catabolic reaction types and the organic functional groups they transform. The purpose of this book is to provide a thorough overview of research and developments being carried out in the field of microbial degradation of xenobiotics compounds which are causing harm to the global environment and human race in general. This book also aims in creating an interest amongst diligent readers who may find new avenues for research. Each section provides an overview of a metabolic pathway of a particular compound that is a central component of the human use in a particular area. Due to its organization, the handbook can be used as a reference book by researchers of related field and can be read for individual sections or chapters. This compilation can also be used as a book for advanced courses in Microbiology, Biochemistry, Biotechnology, Environmental sciences, Enzymology and Metabolic engineering.