This book offers a concise discussion of quantum information in classical gravitational fields described by Einstein's General Theory of Relativity. Besides a basic description of the fundamental physical principles of quantum fields in curved spacetimes, we also offer some new results on steganographic quantum communications in inertial frames, qubits in Schwarzschild spacetime, the spin-curvature coupling in Schwarzschild spacetime, qubits in Kerr spacetimes, and the performance of quantum technologies operating in gravitational fields. A significant part of our discussion is restricted to...