The flux of gas across the air-sea interface is determined by the product of the interfacial concentration difference driving the exchange and a rate constant, often termed the transfer velocity. The concentration-difference term is generally obtained by direct measurement, whereas more indirect approaches are required to estimate the transfer velocity and its variation as a function of controlling parameters such as wind and sea state. Radioactive tracers have proved particularly useful in the estimation of air-sea transfer velocities and, recently, stable purposeful tracers have also started to be used. In this paper the use of the following tracers to determine transfer velocities at the sea surface is discussed: natural and bomb-produced 14 C, dissolved oxygen, 222 Rn and sulphur hexafluoride. Other topics covered include the relation between transfer velocity and wind speed as deduced from tracer and wind-tunnel studies, and the discrepancy between transfer velocities determined by using tracers and from eddy correlation measurements in the atmosphere.