Method of re-lubricating a grease-lubricated bearing arrangement that operates under varying conditions, the method comprising steps of determining a re-lubrication interval t* for the bearing arrangement and initiating a re-lubrication event, based on the determined interval t*, characterised in that the step of determining comprises: - measuring an operating condition C (i) of the bearing at a series of time intervals; - recording an amount of time Δt (i) spent in each operating condition C (i); and either (i) using each operating condition C (i) in the series as an input to a grease life model, to estimate a lubrication interval t lub (i) associated with each operating condition C (i); and using each estimated re-lubrication interval t lub (i) in the series and the associated amount of time Δt (i) to determine a value for t* that is a history- based lubrication interval t lub (t) calculated over the time t since a reference time to, according to the following formula: t lub t = 1 ∑ i = 1 n Δ t i / t t lub i whereby the reference time t 0 is either zero or is the time at which a previous relubrication event occurred; or (ii) associating each operating condition C (i) with a Weibull distribution for grease life; and using the amount of time Δt (i) spent in each operating condition and the Weibull distribution associated with each operating condition C (i) to estimate a history-based reliability value R(t) at the time t since the reference time to, whereby the determined lubrication interval t* is the time at which the estimated reliability value R(t) reaches a predefined minimum value. - measuring an operating condition C (i) of the bearing at a series of time intervals; - recording an amount of time Δt (i) spent in each operating condition C (i); and either (i) using each operating condition C (i) in the series as an input to a grease life model, to estimate a lubrication interval t lub (i) associated with each operating condition C (i); and using each estimated re-lubrication interval t lub (i) in the series and the associated amount of time Δt (i) to determine a value for t* that is a history- based lubrication interval t lub (t) calculated over the time t since a reference time to, according to the following formula: t lub t = 1 ∑ i = 1 n Δ t i / t t lub i whereby the reference time t 0 is either zero or is the time at which a previous relubrication event occurred; or (ii) associating each operating condition C (i) with a Weibull distribution for grease life; and using the amount of time Δt (i) spent in each operating condition and the Weibull distribution associated with each operating condition C (i) to estimate a history-based reliability value R(t) at the time t since the reference time to, whereby the determined lubrication interval t* is the time at which the estimated reliability value R(t) reaches a predefined minimum value. (i) using each operating condition C (i) in the series as an input to a grease life model, to estimate a lubrication interval t lub (i) associated with each operating condition C (i); and using each estimated re-lubrication interval t lub (i) in the series and the associated amount of time Δt (i) to determine a value for t* that is a history- based lubrication interval t lub (t) calculated over the time t since a reference time to, according to the following formula: t lub t = 1 ∑ i = 1 n Δ t i / t t lub i whereby the reference time t 0 is either zero or is the time at which a previous relubrication event occurred; or (ii) associating each operating condition C (i) with a Weibull distribution for grease life; and using the amount of time Δt (i) spent in each operating condition and the Weibull distribution associated with each operating condition C (i) to estimate a history-based reliability value R(t) at the time t since the reference time to, whereby the determined lubrication interval t* is the time at which the estimated reliability value R(t) reaches a predefined minimum value.