This book covers the impact of noise on models that are widely used in science and engineering, and applies perturbed methods which assume noise changes on a faster time or space scale than the system being studied. The book is written in two parts. The first part carefully develops mathematical methods of studying random perturbations of dynamical systems. The second part presents non-random problems, reformulated to account for both external and system random noise, and analyzed using results from Part I. Researchers and graduate students in mathematics and engineering will find this book useful.