Dental appliance compliance monitoring

The present disclosure describes devices, systems, and methods for monitoring use of an oral appliance in an oral cavity using extra-oral sensor(s). One method includes sensing one or more physical properties of an oral appliance using an extra-oral sensor, providing a compliance signal from the extra-oral sensor, identifying one or more patient compliance factors based on the compliance signal, the one or more patient compliance factors providing a basis to identify an extent of compliance of usage of the oral appliance with an orthodontic treatment plan, determining the extent of compliance, and outputting the extent of compliance.