A system for incrementally depositing material includes a delivery system for directing a material toward a deposition zone (5), a monitoring system for monitoring a parameter of the deposited (a) material and a processing system (11) arranged to obtain a monitored value for the parameter, derive a predicted future parameter value for the monitored parameter, compare the predicted value with a reference parameter value for the monitored parameter, and produce a control output based on the comparison of the predicted value with the reference value, the control output being capable of modifying operation of the system. The parameter monitored has the tendency to vary over time (and space), and the processing means ensure that control output is not based upon the difference between an originally monitored parameter value and a reference value but rather between a prediction generated value (accounting for passage of time or spacital difference) and the reference value. This provides for more accurate process control.