The GERDA (GERmanium Detector Array) is an experiment for the search of neutrinoless double beta decay ( 0νββ ) in 76 Ge, located at Laboratori Nazionali del Gran Sasso of INFN (Italy). In the first phase of the experiment, a 90% confidence level (C.L.) sensitivity of 2.4⋅1025 yr on the 0νββ decay half-life was achieved with a 21.6 kg⋅yr exposure and an unprecedented background index in the region of interest of 10 −2 counts/(keV⋅kg⋅yr). No excess of signal events was found, and an experimental lower limit on the half-life of 2.1 ⋅ 10 25 yr (90% C.L.) was established. Correspondingly, the limit on the effective Majorana neutrino mass is mee<0.2–0.4 eV , depending on the considered nuclear matrix element. The previous claim for evidence of a 0νββ decay signal is strongly disfavored, and the field of research is open again.