Quantum Information Theory (QIT) studies how information is processed in the most fundamental level and quantum systems can be manipulated for information applications, and presents the framework of future information technologies. QIT not only deepens our understanding on information processing in the fundamental level but also leads to information-theoretic characterizations to quantum theory. Here, I introduce and discuss fundamentals of QIT such as quantum cloning, classicalquantum correlations, entanglement, non-locality, monogamy of correlations, information causality, etc. These are directly connected to existing and future practical quantum information applications, e.g., cryptography, communication and computation. A general formalism of quantum information processing is provided. I will also discuss the future directions of QIT.