The science of quantum information asks how the laws of nature can be used to manipulate information on a microscopic scale, in ways that have no counterpart in classical physics. Born from a synergy of classical information theory and the foundations of quantum physics, quantum information theory sheds light on fundamental aspects of physics. It continues to provide the theoretical tools needed for the development of quantum technology in the 21st century.Recent decades have seen the rise of quantum information science through what is often called the second quantum revolution. In the current era, the remarkable laws of quantum physics can be used to create new paradigms for information technologies such as computers, communication systems, and sensors. Unlike the first revolution, which relied on quantum effects indirectly, in the second revolution we can engineer quantum mechanics itself to perform tasks. The field is growing at a rapid pace, driving a global race among nations and corporations to develop quantum technologies. In response, universities around the world are increasingly launching courses and programmes dedicated to quantum science and technology. Introduction to Quantum Information Theory is a broad, self-contained, and accessible introduction to the field of quantum information. It serves as an entry point to the topic for students and readers with an academic background in physics or another related field. The focus is on the essential concepts and results, which are supplemented by exercises and detailed solutions.