Classical information can be copied almost without thought. Quantum information cannot. The no-cloning theorem says there is no universal physical operation that perfectly copies an unknown arbitrary state. The reason is not poor engineering; it is linearity. A device that copied $|0⟩$ and $|1⟩$ would not also copy their unknown superposition correctly.

Teleportation is the clean example. The protocol transfers an unknown state using entanglement and two classical bits, but Alice's Bell measurement destroys the original. The result is movement, not duplication. Superdense coding shows the complementary resource accounting: entanglement changes communication capacity without allowing arbitrary state readout.

The no-cloning theorem applies to arbitrary unknown states. Known orthogonal states can be copied because a measurement can distinguish them without ambiguity. No-broadcasting extends the idea to mixed states: noncommuting quantum states cannot be broadcast into two systems with the same local marginals. The no-deleting theorem gives the time-reversed-looking companion: unknown quantum information cannot simply be erased from one copy while preserving another by a universal operation. The Holevo bound adds a measurement-side limit: quantum systems may be described by continuous amplitudes, but the accessible classical information from an ensemble of quantum states is bounded.