The evidence for neutrino flavor oscillations has convincingly shown that neutrino has a finite mass. However, the fundamental question whether neutrino is Majorana or Dirac particle is still unanswered. The only known practical way to probe the Majorana nature of neutrinos experimentally is via the discovery of the neutrinoless double beta (0$\nu\beta\beta$) decay. Moreover, this process violates lepton number conservation. Hence, it is forbidden within the Standard Model of particle physics and, therefore, the discovery of 0$\nu\beta\beta$ decay will confirm the existence of New Physics. That is the reason why during decades, this search remains worldwide ranked amongst the top research priorities. Many 0$\nu\beta\beta$ experiments with a target mass around hundred kg are ongoing and several of next-generation tonne-scale projects are being prepared. The status of some of them is reviewed.