The invention resides in a sensor unit (100) for measuring and monitoring a plurality of parameters associated with an asset or equipment (126, 128) having a fixing connected thereto. The asset can typically be an object, such as a rail or motor casing, that is fixed, using a fixing (122) such as a bolt. The sensor unit has a housing (112) having a portion (120) for receiving a fixing therethrough; sensors (114) arranged to measure and/or monitor (i) parameters associated with a fixing that is secured to the asset via housing and/or (ii) parameters influencing the performance of the asset; a processor (142) configured to process data from the sensors; and a communicator (116) adapted to transmit said data to a remote device. At least one of the sensors can be an inductive sensor, which can measures the displacement or force applied to a fixing against the asset to which it is fixed, said fixing is biased against a resilient member located in the housing. The sensor unit can operate in at least one of a plurality of modes including: a first mode, in which the sensor unit samples one or more parameters periodically at a first sample rate, a second mode, in which the sensor unit detects the occurrence of an event, and a third mode, in which the sensor unit monitors one or more parameters periodically at a second sample rate, which second sample rate is greater than the first sample rate, and wherein the data obtained in said modes is communicable with the system and/or the reader.