There has been a tremendous progress in the mathematical treatment of nonlinear dynamical systems over the past two decades. This book tries to make this progress in the field of stability theory available to scientists and engineers. A unified and systematic treatment of the different types of loss of stability of equilibrium positions of statical and dynamical systems and of periodic solutions of dynamical systems is given by means of the methods of bifurcation and singuality theory. The reader needs only a background in mathematics as it is usually taught to undergraduates in engineering and, having read this book, he should be able to treat nonlinear stability and bifurcation problems himself in a straightforward way. Among others, concepts such as center manifold theory, the method of Ljapunov-Schmidt, normal form theory, unfolding theory, bifurcation diagrams, classifications and bifurcations in symmetric systems are discussed, as far as they are relevant in applications. Most important for the whole representation is a set of examples taken from mechanics and engineering showing the usefulness of the above mentioned concepts. These examples include buckling problems of rods, plates and shells and furthermore the loss of stability of the motion of road and rail vehicles, of a simple robot, and of fluid conveying elastic tubes. With these examples, questions like symmetry breaking, pattern formation, imperfection sensitivity, transition to chaos and correct modelling of systems are touched. Finally a number of selected FORTRAN-routines should encourage the reader to treat his own problem.