Secondary metabolites are organic compounds that are not directly involved in the normal growth, development, or reproduction of an organism. The secondary metabolites are produced majorly by plants and are called phytochemicals, also by microbes such as bacteria, fungi, algae and so on. These secondary metabolites plays a major role in defensive mechanism in plants, as well as its components are used in food industry, pharmaceuticals and so on. The applications and sources of each secondary metabolite is clearly discussed. We are very much thankful for the publisher who readily accepts and publish this subject. Also the author is very much thankful to her research team Mridul Umesh, Thazeem Basheer, Poorna Chandrika Sabapathy, Sabarinathan Devaraj and Sathishkumar Swamiappan for contributing their help and support for this work. The next edition of this book will more precisely discuss on the extraction and purification of the secondary metabolites.