Abstract: According to the present invention, there is provided a method of monitoring a fluid in a fluid carrying conduit comprising the steps of: monitoring a fluid characteristic at a first point and second point along the conduit substantially continuously, determining first and second quantities, being related to a differential with respect to time of the value of the fluid characteristic at the first and second points respectively, combining the first and second quantities to produce a two dimensional intensity function of time and a position variable and analysing the magnitude of the intensity function to derive information relating to the fluid. The intensity function may represent a substantially continuous probability function, wherein its absolute value is related to the probability of a leak or theft having occurred in the conduit.