A method of predicting the condition of a compressor with respect to rotating stall is disclosed. The method comprises the steps of: i) comparing the difference between the largest and smallest values of a variable occurring during a time period with a difference threshold, the variable at any given time being dependent on the compressor exit pressure at that time, and ii) predicting either that the compressor is in rotating stall or that the compressor is not in rotating stall in dependence upon the result of the comparison.