This text is meant to be an introduction to critical point theory and its ap plications to differential equations. It is designed for graduate and postgrad uate students as well as for specialists in the fields of differential equations, variational methods and optimization. Although related material can be the treatment here has the following main purposes: found in other books, To present a survey on existing minimax theorems, To give applications to elliptic differential equations in bounded do mains and periodic second-order ordinary differential equations, To consider the dual...