The increasing complexity of mathematical models, and the related need to introduce simplifying assumptions and numerical approximations, has led to the need to consider approximate solutions. When dealing with any mathematical model, some of the basic questions to be asked are whether the solution is stable to perturbations, what the approximate solutions are, and if the set of approximate solutions is close to the original solution set. The interrelationships between these aspects are also of theoretical interest. Such concepts are described in the present volume, which emphasizes the concepts of approximate solution, well-posedness and stability in optimization, calculus of variations, optimal control, and the mathematics of conflict (e.g. game theory and vector optimization). The most recent developments are covered.