An apparatus for monitoring one or more target objects in an environment external to a host vehicle by means of at least one sensor, the apparatus being arranged to trigger at least one action responsive to the detection of prescribed relative movement between the host vehicle and the one or more target objects. The apparatus is arranged to determine whether the at least one sensor is correctly aligned with respect to the host vehicle whereby, when the apparatus commences monitoring of the environment, the apparatus is arranged not to trigger the at least one action until the apparatus has determined that the at least one sensor is correctly aligned.