Many thousands of years ago, humans were introduced to the toxicants of minerals and plants of microbial origin, and since that time, natural bioactive compounds have been used in traditional medicine for treating different health conditions, but also as aphrodisiacs and as a means of suicide or murder. Modern medicine acknowledges natural bioactive compounds as valuable medicinal sources for both diagnostic and curative purposes. Natural compounds serve as templates for the production of new drugs with improved pharmacological properties. This book explains the term bioactivity and deals with the bioactive compounds of plants, animals, microbial and marine origin. Their use by traditional medicinal approach, as well as by modern medicine is further elaborated. Both beneficial and toxic properties of different chemical classes, including alkaloids, peptides, terpenoids, bioactive amines etc., are described. Their isolation at the industrial scale is presented through several technological processes which are explained in detail for several compound classes. The research explains how natural sources can be exploited my modern and traditional medicine, and presents the risks and benefits associated with their use. In addition, a new approach to studying bioactivity, that includes computational modelling and softwares for in silico description and prediction, is explained. A chemometric approach to studying bioactivity is demonstrated through several models given for some natural bioactive compounds and their derivatives.