Poly-ADP-ribosylation has been proposed to be a reversible protein modification, participating in diverse cellular functions including DNA repair, chromatin remodeling, genetic stability, mitosis, and cell death. Poly-ADP-ribosylation is initiated by the transfer of the ADP-ribose moiety of NAD+ primarily to the carboxyl groups of glutamate and aspartate and amino group of lysine residues in target proteins, followed by elongation of poly(ADP-ribose) (PAR) chains via α-O-glycosidic (C- 1"-C-2') ribose-ribose bonds. PAR consists of polymers of ADP-ribose (up to 200 units) with branching via α-O-glycosidic (C-1"'-C-2") ribose-ribose bonds. Further, the pyrophosphate group of each ADP-ribose has two negative charges. Therefore, in proteins modified by PAR, a complex structure with negative charges may lead to dynamic changes of functions. PAR formation is catalyzed by poly(ADP-ribose) polymerases (PARPs) and terminated by several types of enzymes with PAR-degrading activities; poly(ADP-ribose) glycohydrolase (PARG), ADP-ribosylacceptor hydrolase (ARH) 3, ARH1, and macrodomain-containing proteins. PARG has been thought to be primarily responsible for PAR degradation. In 2006, ARH3 was cloned and identified as another type of PAR-degrading protein. Although PAR-degrading activity of ARH3 is less than that of PARG, different mechanisms of PAR recognition and the cellular localization of ARH3 appear to be responsible for unique cellular roles of ARH3 involving PAR. In the present review, we focused on our findings regarding structure, biological properties, and cellular functions of ARH3. In addition, we describe the current knowledge of poly-ADP-ribosylation and cell death pathways regulated PARP1, PARG, and ARH3.