Although it has been known for almost half a century that migratory birds can 
detect the direction of the Earth's magnetic field, the primary sensory 
mechanism behind this remarkable feat is still unclear. The leading hypothesis 
centers on radical pairs-magnetically sensitive chemical intermediates formed by 
photoexcitation of cryptochrome proteins in the retina. Our primary aim here is 
to explain the chemical and physical aspects of the radical-pair mechanism to 
biologists and the biological and chemical aspects to physicists. In doing so, 
we review the current state of knowledge on magnetoreception mechanisms. We dare 
to hope that this tutorial will stimulate new interdisciplinary experimental and 
theoretical work that will shed much-needed additional light on this fascinating 
problem in sensory biology.