  Quantum dense coding is one of the most important protocols in quantum
communication. It derives from the idea of using quantum resources to boost the
communication capacity and now serves as a key primitive across a variety of
quantum information protocols. Here, we focus on the basic theoretical ideas
behind quantum dense coding, discussing its development history from discrete
and continuous variables to quantum networks, then to its variant protocols and
applications in quantum secure communication. With this basic background in
hand, we then review the main experimental achievements, from photonic qubits
and qudits to optical modes, nuclear magnetic resonance, and atomic systems.
Besides the state of the art, we finally discuss potential future steps.
