This chapter provides a comprehensive overview of Rydberg atoms, with emphasis on the preparation methods, physical properties, and their extraordinary long-range interactions. Both resonant dipoledipole and van der Waals couplings are treated in detail, including the Frster resonance picture and angular dependence of interaction strength. Mechanisms for controlling interactions via electromagnetic fields and distance-selective dressing are presented, highlighting the central role of Rydberg blockade and tunability in many-body quantum dynamics and quantum information protocols.