Proton magnetometers are well known as precise instruments for the measurement of totalmagnetic field. Such position is based on the proton gyromagnetic ratio, a fundamental physical constant. In this report we discuss some general metrological aspects of the proton and Overhauser magnetometers which can be used for development and exploitation. This discussion is stimulated by researches of the new Overhauser magnetometer POS-1 that is intended for magnetic observatories and fieldwork.