The invention provides a method and a system for monitoring of yarn drawn axially from a bobbin (24a). It is desired to monitor one or both of remaining capacity of an active bobbin and transfer from one active bobbin 24a to the next. A free portion (30) of the yarn moves circumferentially about the bobbin as the yarn is drawn from it. The invention involves a sensor responsive to electromagnetic radiation arranged to sense the free portion (30) of the yarn and to provide an output which varies with a period P corresponding to the period of the circumferential movement of the free portion of the yarn about the bobbin (24a). The period P can be interpreted to provide the desired information.