The aim of this paper is to study the state of the art about the metrological characteristics and the accuracy of wearable devices, tested in comparison to a gold standard instrument. A bibliographic research has been made on the main scientific databases (e.g. Scopus and Web of Science). Papers have been included on the basis of established criteria (e.g. the wearable device has to be commercial). At present, neither a standard protocol nor fixed metrological characteristics can be identified in the literature. Among the most discussed wearable devices, there are certainly Fitbit, Jawbone, Garmin and Polar ones. Chest-strap monitors generally result to be more accurate than wrist-worn devices, which, on the other hand, are cheaper and more comfortable. Given the lack of standards in the validation process, the data appear to be very irregular (even among studies conducted on the same device) and consequently barely comparable. It would be extremely important to conduct a pilot study on a few devices, validating them according to an established test protocol and comparing the results to a gold reference instrument (e.g. ECG for Heart Rate assessment). In this way, it would be possible to start building a database of the accuracy and the metrological characteristics of wearable devices.