  The GERmanium Detector Array, GERDA, is designed to search for neutrinoless
double beta (0$\nu\beta\beta$) decay of $^{76}$Ge and it is installed in the
Laboratori Nazionali del Gran Sasso (LNGS) of INFN, Italy. In this review, the
detection principle and detector setup of GERDA are described. Also, the main
physics results by GERDA Phase I, are discussed. They include the measurement
of the half-life of 2$\nu\beta\beta$ decay, the background decomposition of the
energy spectrum and the techniques for the discrimination of the background,
based on the pulse shape of the signal. In the last part of this paper, the
estimation of a limit on the half-life of 0$\nu\beta\beta$ ($T^{0\nu}_{1/2}
>2.1\cdot 10^{25}$ yr at 90% C.L.) and the comparison with previous results are
discussed. GERDA data from Phase I strongly disfavour the recent claim of
0$\nu\beta\beta$ discovery, based on data from the Heidelberg-Moscow
experiment.
