Seven years worth of meteorological tower data from the site Östergarnsholm in the Baltic Sea have provided new information on the structure of the marine atmospheric boundary layer,of high relevance for off-shore wind energy siting. The present paper is a review based on a series of original papers, w hich have appeared recently or are about to appear in the meteorological literature. Waves on the water surface modify the flow considerably as soon as they become of sufficient length. Thus, during neutral conditions, a logarithmic wind profile is observed only w hen the waves are growing actively. During unstable stratification, w hich occurs at this site during more than half the time, several factors act together to reduce the magnitude of the wind shear to a considerable extent. Velocity spectra during neutral conditions and growing sea agree with corresponding spectra over land, which have recently been show n to have appreciably more low-frequency energy than previously assumed. The marine spectra are shown to be strongly dependent on wave-state. The effect of limited water depth on the structure of the marine surface layer is discussed.