In this first volume manifolds with boundary and critical point theory play an important role. We focus our attention to topological aspects in nonlinear optimization. In this way, we do not treat numerical and algorithmic features; however, we believe that the present work will provide a good insight into nonlinear phenomenae and that it will be helpful in order to device computational strategies towards global optimization. In order to underline the geometry of our analysis, many figures are included. In a second volume, we treat structural stability, transversality, (gradient-, Newton-)flows and parametric aspects.