In this introductory chapter, we consider the concept on differentiability of mappings in Banach spaces, Frechet and Gâteaux derivatives, secondorder derivatives and general minimization theorems. Variational principles of Ekeland [Ek1] and Borwein & Preiss [BP] are proved and relations to the minimization problem are given. Deformation lemmata, Palais—Smale conditions and mountain-pass theorems are considered. The deformation approach and e—variational approach are applied to prove the mountainpass theorem and its various extensions.