A tensor category is a category with a tensor product that satisfies properties similar to the tensor product of vector spaces. A typical example is given by the representation category of a group with the usual tensor product. While the most important examples of tensor categories arise from representation theory, the theory has outgrown its origins and has emerged into a vast and complex theory on its own, providing a unified language for many phenomena in different fields. Tensor categories are now ubiquitous in areas such as representation theory, invariants of links and 3-manifolds, algebraic geometry, higher category theory, quantum computing and mathematical physics. All in all the abstract theory of tensor categories along with its applications has seen a rapid growth over the last 10 - 20 years.