  Many experiments have been carried out to study the beta-decay rates of a
variety of nuclides, and many - but not all - of these experiments yield
evidence of variability of these rates. While there is as yet no accepted
theory to explain patterns in the results, a number of conjectures have been
proposed. We discuss three prominent conjectures (which are not mutually
exclusive) - that variability of beta-decay rates may be due to (a)
environmental influences, (b) solar neutrinos, and (c) cosmic neutrinos. We
find evidence in support of each of these conjectures.
