A method for predicting remaining useful life of a wind or water turbine or component thereof, the method comprising the steps of: determining an EOH for the wind or water turbine or component thereof according to the steps of: providing data relating to one or more operating conditions; and providing one or more EOH coefficients (106,108,220) relating to the one or more operating conditions; wherein the EOH is a function of the data relating to the one or more operating conditions and the one or more EOH coefficients (106,108,220) relating to the one or more operating conditions; obtaining an EOH limit value for the wind or water turbine or component thereof; and comparing the EOH and the EOH limit, wherein the comparison of EOH limit and EOH yields a value for the remaining useful life of the wind or water turbine or component thereof, wherein the EOH coefficients (106, 108, 220) are obtained from assessing damage to the wind or water turbine or a component thereof under field operating conditions and damage to the wind or water turbine or a component thereof under rated operating conditions of the same duration, wherein the component is a gearbox, wherein assessing damage includes: monitoring outputs of one or more condition monitoring sensors placed in or on the gearbox at predetermined locations; and based on said outputs, calculating damage using a meta-model of the gearbox, the meta-model being constructed by obtaining a number of data samples from a gearbox model prior to the start of gearbox operation and determining an underlying trend using response surface methodology. determining an EOH for the wind or water turbine or component thereof according to the steps of: providing data relating to one or more operating conditions; and providing one or more EOH coefficients (106,108,220) relating to the one or more operating conditions; wherein the EOH is a function of the data relating to the one or more operating conditions and the one or more EOH coefficients (106,108,220) relating to the one or more operating conditions; providing data relating to one or more operating conditions; and providing one or more EOH coefficients (106,108,220) relating to the one or more operating conditions; wherein the EOH is a function of the data relating to the one or more operating conditions and the one or more EOH coefficients (106,108,220) relating to the one or more operating conditions; obtaining an EOH limit value for the wind or water turbine or component thereof; and comparing the EOH and the EOH limit, wherein the comparison of EOH limit and EOH yields a value for the remaining useful life of the wind or water turbine or component thereof, wherein the EOH coefficients (106, 108, 220) are obtained from assessing damage to the wind or water turbine or a component thereof under field operating conditions and damage to the wind or water turbine or a component thereof under rated operating conditions of the same duration, wherein the component is a gearbox, wherein assessing damage includes: monitoring outputs of one or more condition monitoring sensors placed in or on the gearbox at predetermined locations; and based on said outputs, calculating damage using a meta-model of the gearbox, the meta-model being constructed by obtaining a number of data samples from a gearbox model prior to the start of gearbox operation and determining an underlying trend using response surface methodology. monitoring outputs of one or more condition monitoring sensors placed in or on the gearbox at predetermined locations; and based on said outputs, calculating damage using a meta-model of the gearbox, the meta-model being constructed by obtaining a number of data samples from a gearbox model prior to the start of gearbox operation and determining an underlying trend using response surface methodology.