Quantum Field Theory is the basic pillar of our description of nature. Every phenomenon we observe has, at its core, an underlying quantum field theory behind.áTherefore, it is impossible to overstate its importance. This book represents the first encounter that undergraduate students at the Autonomous University of Barcelona have with this subject. Itáis largely based on the lecture notes from an introductory course on quantum field theories taught during the final semester of the undergraduate Physics program. Experience shows that the material presented in this book more than covers the scope of a one-semester course. The approach taken in this book differs in some aspects from traditional quantum field theory textbooks.á These often fail to provide a smooth connection to previous courses in non-relativistic quantum mechanics or classical field theory, even though they should naturally emerge as limiting cases of relativistic quantum field theories. In contrast, these lecture notes make a deliberate effort to build that bridge?particularly with non-relativistic quantum mechanics. At the same time, this book strives to remain truly introductory, paying close attention to detail and resisting the temptation to become overly broad.