During the past quarter century the study of air-sea interaction has e
volved from a small branch of marine climatology to play a key role in
the modelling of the coupled system of ocean and atmosphere. Knowledg
e of air-sea fluxes has grown, based on Monin-Obukhov similarity theor
y for surface boundary layers and on direct and indirect techniques of
measuring the fluxes. This has been the basis for providing boundary
conditions needed to couple atmospheric and oceanic circulation models
that are used to forecast weather and climate. An overview of current
understanding is followed by a discussion of parameterisation schemes
and a chronicle of some of the experimental work that has tested theo
ries and quantified their conclusions.