1. A method of intrusion detection, the method including: displaying sensing device output data as one or more images of a field of view of a sensing device; defining at least one intrusion detection criterion via a graphical user interface by: receiving user input to define and display an image detection area in relation to the one or more images of the field of view, wherein the image detection area is correlated to a detection area to be monitored by the sensing device for intrusion detection; and receiving user input that defines a detection vector associated with the detection area, said detection vector specifying a predetermined minimum displacement and direction of displacement of a potential intruder in the field of view to be monitored, displaying a graphical representation of the detection vector on the image detection area; detecting a target object; and tracking the target object at least partially through the detection area and determining whether the progressive displacement of the target object has a component which exceeds the minimum displacement in the direction of the detection vector.