The combination of an ultra-low background environment with active background suppression techniques allows GERDA (GERmanium Detector Array) to perform a background-free search for neutrinoless double beta decay of $^{76}$Ge. GERDA is located at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN in Italy. In Phase II 35.6 kg of isotopically enriched high purity germanium detectors are operated in a liquid argon bath. The background index in the region-of-interest is $<10^{-3}$ counts/(keV$\cdot$kg$\cdot$yr). No signal has been found in the data corresponding to 58.9 kg$\cdot$yr of exposure and in combination with data from Phase I a new lower limit on the half-life of neutrinoless double beta decay in $^{76}$Ge of T$_{1/2}>0.9\cdot10^{26}$ yr (90% C.L.) has been established. GERDA is the first neutrinoless double beta decay experiment that surpassed a half-life sensitivity of $>10^{26}$ yr.