It is shown that a surface wave will be generated by a horizontal slot situated above a corrugated or dielectric-coated guiding surface.A physical interpretation of the analysis is suggested which explains the mechanism of launching in terms of a resonance phenomenon allied to that which arises when a piston attenuator is terminated by a reactance equal and opposite to its own (reactive) characteristic impedance.The problem of efficient launching is discussed, and it is shown that under suitable conditions a high launching efficiency is possible.Some of the theoretical predictions have been verified experimentally.