Euler –Lagrange equations and solutions of partial differential equations via the Dirichlet principle of minimal Energy. Towards weak solutions. A few facts from Functional Analysis: weak derivatives and Sobolev spaces, embedding inequalities, the Rellich-Kondrachov theorem, extensions and traces. A direct method in the Calculus of Variations, minima of weakly lower semicontinuous functionals: applications to nonlinear Schroedinger’s equation. The Nehari manifold and ground states solutions, bootstrap argument in elliptic regularity theory. Introduction to topological methods in Nonlinear Analysis for indefinite functionals: deformation lemma and the mountain-pass theorem by Ambrosetti-Rabinowitz, applications to semilinear elliptic equations. The Ekeland Variational Principle. The effect of Symmetry, Critical growth problems, lack of compactness and Pohozaev identity. Quantization of energy and the Brezis-Nirenberg theorem.