**3. Neutrinoless Double Beta Decay Nuclear Matrix Elements**

From previous Sections, one can conclude that the analysis of main experimental data regarding the 0*νββ*, the half-lives of multiple isotopes, and the two-electron angular and energy distributions [15] require a set of nuclear matrix elements. In this Section, I describe different techniques for calculating NMEs, starting with the direct summation on the states in the intermediate nucleus (*Z* − 1, *N* − 1), where *Z* denotes the atomic number and *N* the number of neutrons in a nucleus, and continuing with the often used closure approximation. An alternative method that performs a summation on the intermediate states in the (*Z* − 2, *N*) or (*Z*, *N* − 2) nuclei is described in Ref. [46].
