A method of monitoring the pressure of a tire of an aircraft, the method including: taking two or more pressure readings from the tire at different times; calculating an estimated deflation rate based on the pressure readings; and calculating a time for the tire to deflate to a reference pressure level based on the estimated deflation rate. Two or more temperature readings are each associated with one of the pressure readings, and the estimated deflation rate is calculated by normalising each pressure reading based on its associated temperature reading and a common reference temperature to obtain a temperature-normalised pressure reading, and calculating the estimated deflation rate based on the temperature-normalised pressure readings. The estimated deflation rate is compared with a threshold, and a warning provided if the estimated deflation rate exceeds the threshold.