Quantum mechanics carries with it extra features, such as superposition, entanglement, and interference, that are not present in classical mechanical systems. As a result, quantum information processing will lead to new features that cannot be mimicked by classical information processing systems. So classical information measures have to be generalized in order to capture these new features. These new quantum information measures should reduce back to the old ones as limiting cases. In a later chapter, it will be seen that quantum measures of information are closely related to quantitative measures of entanglement, and so in addition to limits on communication rates and computation rates, there will also be limits on rates of entanglement sharing.