There are 35 naturally occurring isotopes capable of double beta decay. In practice, the decay can be observed when the single beta decay is forbidden by energy conservation. This happens for elements with an even atomic number and even neutron number, which are more stable due to spin-coupling. When single beta decay or alpha decay also occur, the double beta decay rate is generally too low to observe. However, the double beta decay of  (also an alpha emitter) has been measured radiochemically. Two other nuclides in which double beta decay has been observed,  and , can also theoretically single beta decay, but this decay is extremely suppressed and has never been observed. Similar suppression of energetically barely possible single beta decay occurs for Gd and Rn, but both these nuclides are rather short-lived alpha emitters.
Fourteen isotopes have been experimentally observed undergoing two-neutrino double beta decay (ββ) or double electron capture (εε). The table below contains nuclides with the latest experimentally measured half-lives, as of December 2016, except for Xe (for which double electron capture was first observed in 2019). Where two uncertainties are specified, the first one is statistical uncertainty and the second is systematic.
Searches for double beta decay in isotopes that present significantly greater experimental challenges are ongoing. One such isotope is  .
The following known beta-stable (or almost beta-stable in the cases Ca, Zr, and Rn) nuclides with A ≤ 260 are theoretically capable of double beta decay, where red are isotopes that have a double-beta rate measured experimentally and black have yet to be measured experimentally: Ca, , Zn, , Se, , Kr, Zr, , Mo, , Ru, Pd, Cd, , Sn, Sn, , , Xe, , Ce, Nd, Nd, , Sm, Gd, Er, Yb, W, Os, Pt, Hg, Po, Rn, Rn, Ra, Th, , Pu, Cm, Cf, Cf, and Fm.
The following known beta-stable (or almost beta-stable in the case Gd) nuclides with A ≤ 260 are theoretically capable of double electron capture, where red are isotopes that have a double-electron capture rate measured and black have yet to be measured experimentally: Ar, Ca, Cr, Fe, Ni, Zn, Se, , Sr, Mo, Ru, Pd, Cd, Cd, Sn, Te, , Xe, , Ba, Ce, Ce, Sm, Gd, Gd, Gd, Dy, Dy, Dy, Er, Er, Yb, Hf, W, Os, Pt, Hg, Rn, Rn, Ra, Th, U, Pu, Cm, Fm, and No.
In particular, Ar is the lightest observationally stable nuclide whose decay is energetically possible.