A method of countering the rolling motion of a marine vessel , the marine vessel comprising one or more active stabilisers , the method comprising the steps of: sensing a speed and a heading for the marine vessel relative to the approaching wave; characterising a first surface profile of a wave field approaching the marine vessel , using one or more sensors ; predicting a second surface profile of the wave field resulting from the wave field encountering the marine vessel at a chosen time in the future, using the characterised first surface profile , together with the vessel's speed and heading; predicting a first impingement time being the time at which the approaching wave field will impinge on the marine vessel ; applying a harmonic analysis technique to decompose the first surface profile into constituent simple sinusoidal wave components of known amplitude and length; predicting the wave celerity of the constituent sine waves; determining the relative phase shift of the sinusoidal wave components; and phase shift the component waves for the future impingement of the wave field on the marine vessel for the first infringement time and then recombine the constituent wave components in their shifted relationship to derive a new wave slope corresponding to the future impingement of the wave field on the marine vessel ; computing one or more control signals, using the predicted second surface profile ; and transmitting the one or more control signals to the one or more active stabilisers , to cause the one or more active stabilisers to apply a heeling moment to the marine vessel to counter the roll moment produced by the impingement of the wave field on the marine vessel . sensing a speed and a heading for the marine vessel relative to the approaching wave; characterising a first surface profile of a wave field approaching the marine vessel , using one or more sensors ; predicting a second surface profile of the wave field resulting from the wave field encountering the marine vessel at a chosen time in the future, using the characterised first surface profile , together with the vessel's speed and heading; predicting a first impingement time being the time at which the approaching wave field will impinge on the marine vessel ; applying a harmonic analysis technique to decompose the first surface profile into constituent simple sinusoidal wave components of known amplitude and length; predicting the wave celerity of the constituent sine waves; determining the relative phase shift of the sinusoidal wave components; and phase shift the component waves for the future impingement of the wave field on the marine vessel for the first infringement time and then recombine the constituent wave components in their shifted relationship to derive a new wave slope corresponding to the future impingement of the wave field on the marine vessel ; computing one or more control signals, using the predicted second surface profile ; and transmitting the one or more control signals to the one or more active stabilisers , to cause the one or more active stabilisers to apply a heeling moment to the marine vessel to counter the roll moment produced by the impingement of the wave field on the marine vessel . <EOS>