Introduction.- Atomic Structure and Atom-light Interactions.- Experimental Methods.- Probing an Excited State Transition Using Quantum Beats.- Intrinsic Rydberg Optical Bistability.-Terahertz Electrometry with Rydberg EIT.- Real-time Near-field Terahertz Field Imaging.-Terahertz-driven Phase Transition in a Hot Rydberg Vapour.- Summary and Outlook.