This paper reviews the contributions of transient tracer research to our understanding of the lower limb of the thermohaline circulation in the North Atlantic Ocean. The review is limited geographically to the North Atlantic because of its importance for decadal scale climate variability. The tracers reviewed are the transient tracers of anthropogenic origin that give decadal time scale information, the chlorofluorocarbons (CFCs) and tritium/3He. The emphasis is on work by United States (US) investigators from 1991–94. Also included are some non-US and earlier contributions as they are linked to the recent work and have not been reviewed in this publication before.