A method of estimating the instantaneous crop yield y.sub.2(t) at time t in a field, during harvesting with a harvesting machine that operates by cutting and/or controlling the crop and has a crop mass flow sensor that measures the mass flow rate of crop processed by the machine at a location remote from that at which the harvesting machine cuts the crop and generates a signal {dot over ({circumflex over (m)}.sub.out(t) indicative thereof, including the steps of: generating a mass flow rate signal {dot over ({circumflex over (m)}.sub.out(t) using the sensor;generating an area rate signal s(t) indicative of time domain variations in the area of the field harvested per time unit;filtering the area rate signal s(t) using a function P(s) representative of the dynamics of the harvesting machine to generate a filtered area rate signal s.sub.2(t); andusing the filtered area rate signal s.sub.2(t) for deriving from the mass flow rate signal {dot over ({circumflex over (m)}.sub.out(t) a yield per area unit y.sub.2(t).