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Blockade interactions whereby a single particle prevents the flow or excitation of other particles provide a mechanism for control of quantum states, including entanglement of two or more particles. Blockade has been observed for electrons^,,, photons and cold atoms. Furthermore, dipolar interactions between highly excited atoms have been proposed as a mechanism for ‘Rydberg blockade’^,, which might provide a novel approach to a number of quantum protocols^,,,. Dipolar interactions between Rydberg atoms were observed several decades ago and have been studied recently in a many-body regime using cold atoms^,,,,,. However, to harness Rydberg blockade for controlled quantum dynamics, it is necessary to achieve strong interactions between single pairs of atoms. Here, we demonstrate that a single Rydberg-excited rubidium atom blocks excitation of a second atom located more than 10 μm away. The …