It was recently discovered that the photoreceptor cryptochrome is involved in mediating magnetosensitive entrainment of the internal clock of fruit flies.1 This discovery follows other recent studies implicating a role of cryptochrome in mediating magnetic sensitivity in orientation responses of fruit flies2,3 and growth responses of plants.4 Such widespread use of the same molecule for mediating magnetic sensitivity might suggest that cryptochrome is in some way optimal for detecting the magnetic field of the earth and that the magnetoreception function cannot be easily taken over by other molecules. This raises the question what properties might set cryptochromes apart from other molecules in terms of their ability to detect the geomagnetic field. Here, we will discuss possible answers to this question. We will first review the likely biophysical mechanism by which magnetic fields can be detected by a photoreceptor and discuss what constitutes an optimal photo-magneto-receptor. We will then discuss in how far cryptochrome matches the profile of an optimal molecule and what further steps are required for more conclusive answers.