Abstract: An apparatus for controlling the concentration of toner within a developer material of carrier and toner. The apparatus having a control means for generating a toner addition signal indicative of the amount of toner to be added to the developer material. The control means including the ability to measure the concentration of toner within the developer material during at least a first period and a second period subsequent to the first period. The control means also determining a first concentration error as a function of the deviation between the toner concentration measured during the first period and a reference toner concentration and a second concentration error as a function of the deviation between the toner concentration measured during the second period and the reference toner concentration. Subsequently, the control means generates the toner addition signal as a function of the first and second concentration error values.