The DAFNE electron-positron collider of the Laboratori Nazionali di Frascati of INFN is a worldwide unique low-energy kaon source, which is being used to produce and to study kaonic atoms by the SIDDHARTA collaboration. The X-ray measurements of kaonic atoms play an important role for understanding the low-energy QCD in the strangeness sector. Significant achievements have been obtained by the SIDDHARTA experiment, among which: the most precise kaonic hydrogen measurement of the 1s level shift and width induced by the presence of the strong interaction; an upper limit of the X-ray yield for kaonic deuterium K-series; the accurate measurement of the 2p level shift and width of kaonic helium-4 and kaonic helium-3; yields of various light kaonic atoms transitions. Using the experience gained with SIDDHARTA experiment, the first kaonic deuterium measurement is in preparation in the framework of the SIDDHARTA-2 experiment with the goal to determine the antikaon-nucleon isospin dependent scattering lengths, which is possible only by combining the K-p and the upcoming K-d results. An overview of the experimental results of SIDDHARTA and an outlook to SIDDHARTA-2 experiments are given in this paper.