This paper presents a synthesis of non-linearities present in mechatronic air path actuators of diesel engines. These actuators allow precise control over air path dynamics because of their fast response times and accuracy in transient conditions. However, the non-linearities present in their characteristics limit their closed-loop performance. In this paper we discuss various non-linear phenomena existing in actuator characteristics and their physical causes. Different modelling schemes for the physical causes are also presented, which simulate the non-linear behaviour of the actuators. Furthermore, other factors, such as operational conditions and ageing, and their degrading effects on the actuator performance, are also discussed.