A wearable device may include a sensor system capable of obtaining physiological from a user's body. Some wearable devices may include a substance delivery system. A sensor system of a wearable device may include at least one “bio-assurance sensor” capable of obtaining biometric data that may be used to identify a user. For example, the bio-assurance sensor may be used to ensure that the wearable device is not removed from the user's body and/or placed on or in another user's body. In some examples, the wearable device may be used with a second device, such as a smart phone, that includes at least one “authentication sensor,” such as a fingerprint sensor, that also may be used to identify a user. However, in some implementations the wearable device may include at least one authentication sensor.