The collective behavior of quantum and classical many body systems such as ultracold atomic gases, nanowires, cuprates and micromagnets are currently subject of an intense experimental and theoretical research worldwide. Understanding the fascinating phenomena of Bose-Einstein condensation, Luttinger liquid vs non-Luttinger liquid behavior, high temperature superconductivity, and spontaneous formation of periodic patterns in magnetic systems, is an exciting challenge for theoreticians. Most of these phenomena are still far from being fully understood, even from a heuristic point of view. Unveiling the exotic properties of such systems by rigorous mathematical analysis is an important and difficult challenge for mathematical physics. In the last two decades, substantial progress has been made on various aspects of many-body theory, including Fermi liquids, Luttinger liquids, perturbed Ising models at criticality, bosonization, trapped Bose gases and spontaneous formation of periodic patterns. The techniques successfully employed in this field are diverse, and range from constructive renormalization group to functional variational estimates. In this research project we propose to investigate a number of statistical mechanics models by a combination of different mathematical methods. The objective is, on the one hand, to understand crossover phenomena, phase transitions and low-temperature states with broken symmetry, which are of interest in the theory of condensed matter and that we believe to be accessible to the currently available methods; on the other, to develop new techiques combining different and complementary methods, such as multiscale analysis and localization bounds, or reflection positivity and cluster expansion, which may be useful to further progress on important open problems, such as Bose-Einstein condensation, conformal invariance in non-integrable models, existence of magnetic or superconducting long range order.