Gauge invariance is a powerful tool to determine the dynamics of the
electroweak and strong forces. The particle content, structure and symmetries
of the Standard Model Lagrangian are discussed. Special emphasis is given to
the many phenomenological tests which have established this theoretical
framework as the Standard Theory of electroweak interactions.Comment: Lectures given at the 2004 European School of High-Energy Physics,
Sant Feliu de Guixols, Spain, 31 May - 11 June 2004 47 pages, 35 figure