Neutrinos play prominent roles in both particle physics and cosmology. In this talk, I will cover two broad topics. The first will be possible origins for neutrino masses and mixings and the implications of this physics for cosmology. Some non‐cosmological digressions on the flavour problem in general will be made. The second topic will be Big Bang Nucleosynthesis (BBN) and bounds on active‐sterile neutrino mixing. © 2003 American Institute of Physics