The mouth-guard has a body 12 suitable to extend around a portion of a tooth of a wearer and has a system 28 for monitoring acceleration experienced by the mouth-guard. The system 28 communicates acceleration data to a monitoring station. The system 28 and a power source 26 are embedded within material of the body and are located approximate a posterior teeth region of a mouth of a wearer. The power source 26 and system 28 may be embedded within a palatal wall 16 of the body 12 and may be encapsulated in an inert material, e.g. parylene C. An antenna may be remote from the system 28 and may be part of a connection lead between the power source 26 and system 28. The acceleration detected may relate to head impacts in contact sports and be used to monitor for concussion. Also provided is a mouth device for measuring physiological data which may include a receiver to communicate received data from a remote device to monitoring circuitry of the mouth device. The physiological data may be hydration, temperature or electrolyte levels.
