We have attempted to explain the concepts which have been used and developed to model the stochastic dynamics of natural and biological systems. While the theory of stochastic differential equations and stochastic processes provide an attractive framework with an intuitive appeal to many problems with naturally induced variations, the solutions to such models are an active area of research, which is in its infancy. Therefore, this book should provide a large number of areas to research further. We also tried to explain the ideas in an intuitive and descriptive manner without being mathematically rigorous. Hopefully this will help the understanding of the concepts discussed here