Neutrino-mass models in extra dimensions

Neutrinos play a crucial role in many areas of physics, from particle physics, at very short distances to astrophysics and cosmology, It is a long-held belief that they are good probes of physics at the GUT scale. Recent developments have made it clear that they can also be of fundamental importance for the physics of extra dimensions, if these exist. Here, we pedagogically review the construction of neutrino-mass models in extra dimensions, within the brane-world scenarios. These models are usually nontrivial generalizations of their four-dimensional counterparts. We describe the theoretical tools that have been forged and the new perspectives gained in this rapidly developing area. In particular, we discuss the issues involved in building models without the use of right-handed singlets. It is very difficult to directly test the origin of neutrino masses in the different models, be it in four or more dimensions. We point out that different models give very different indirect signatures in the TeV region and precision measurements.