This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book highlights quantum field theory techniques that were developed in parallel in high energy physics and condensed matter physics. It explains the relevance of quantum field theory methods developed in the context of high energy physics, to many particle systems, even though, in principle, quantum mechanics may suffice to understand these systems. The book discusses the modern perspective of phase transitions and use a length scale based approach. It deals with the application of the ideas of bosonization in simple one-dimensional models such as spin models and the Hubbard model, where the calculational tools of bosonization and Renormalisation group are used to compute physically relevant quantities like conductances. The book also highlights the idea that topological concepts play an important role in a wide range of phenomena.