The book gathers the lecture notes of the Les Houches Summer School that was held in August 2011 for an audience of advanced graduate students and post-doctoral fellows in particle physics, theoretical physics, and cosmology, areas where new experimental results were on the verge of beingdiscovered at CERN. Every Les Houches School has its own distinct character. This one was held during a summer of great anticipation that at any moment contact might be made with the most recent theories of the nature of the fundamental forces and the structure of space-time. In fact, during the session, the longanticipated discovery of the Higgs particle was announced. The book vividly describes the fruitful and healthy "schizophrenia" that is the rule among the community of theoreticians who have split into several components: those doing phenomenology, and those dealing with highly theoretical problems,with a few trying to bridge both domains. The lectures by theoreticians covered many directions in the theory of elementary particles, from classics such as the Supersymmetric Standard Model to very recent ideas such as the relation between black holes, hydrodynamics, and gauge-gravity duality. The lectures by experimentalists explained indetail how intensively and how precisely the LHC collider has verified the theoretical predictions of the Standard Model, predictions that were at the front lines of experimental discovery during the 70's, 80's and 90's, and how the LHC is ready to make new discoveries. They described many of theingenious and pioneering techniques developed at CERN for the detection and the data analysis of billions of billions of proton-proton collisions.