Poly(ADP-ribose) polymerase 1 (PARP1), is a 113 kDa nuclear protein coded by the PARP1 gene on chromosome 1q42.12.  PARP1 is the first detected and most abundant member of the PARP superfamily. It modulates the activity of DNA binding proteins by catalyzing their Poly(ADP-ribosyl)ation (PARylation), a post-translational modification affecting the conformation and function of affected proteins. PARP1 impacts several DNA repair processes including the pathways of nucleotide excision repair, non-homologous end joining, microhomology-mediated end joining, homologous recombinational repair, and DNA mismatch repair. PARP1 detects DNA damage and then modulates repair efficiency by ADP-ribosylation of histones, subsequent decompaction of chromatin structure, and through interaction with and modification of multiple DNA repair factors. PARP1 and PARylation also play a role in a wide range of further cellular processes such as cell death, chromatin remodeling, inflammatory response and gene transcription which can be independent of DNA damage response. PARP1 and PARylation homeostasis have also been implicated in multiple diseases, including inflammation, stroke, diabetes and cancer. Several PARP1 inhibitors have been approved as cancer drugs and they are successfully applied in an increasing number of different cancer types.