In this introductory talk we will look at many body physics by means of the partition function as its central object. Starting with an introduction to tensor networks as a general formalism we will use the language of tensor networks in order to get an intuition for the partition function and hence many body physics. The notion of equivalence classes of tensor networks will lead us directly to a notion of equivalence classes of many body systems usually referred to as phases. As a special case we will dig deeper into the equivalence class with respect to topological moves (Pachner moves) leading directly to gapped topological phases. Combining the properties of tensor networks and topological moves we will introduce the entanglement renormalization scheme as a method to detect topological phases. We will conclude the talk with some general remarks on renormalization, regularization and discretization.