Accumulating evidence has suggested the fundamental functions of NAD+-poly(ADP-ribose) metabolism in cellular and physiological processes, including energy homeostasis, signal transduction, DNA transaction, genomic stability and cell death or survival. The NAD+ biosynthesis and poly(ADP-ribose) [(ADP-R)n] turnover are tightly controlled by several key enzymes, such as nicotinamide phosphoribosyltransferase (NmPRT), nicotinamide mononucleotide adenylyltransferases (NMNATs), poly(ADP-ribose) polymerase (PARP), poly(ADP-ribose) glycohydrolase (PARG) and ADP-ribose pyrophosphorylase (ADPRPPL). Many researches investigating the roles of these enzymes in cells have revealed the physiological and pathological importance, and thereby the therapeutical values. Among these enzymes, the polymer degrading enzyme PARG has not yet been intensively studied, because of the low cellular content, lack of cell-available PARG chemical inhibitors and PARG genetic models. So, the biological roles of (ADP-R)n catabolism by PARG are still being elucidated as compared to those of synthesis by PARP. However, recent studies delineate that PARG-dependent (ADP-R)n degradation is critical for many pathological conditions, and thus PARG is an important target for chemical therapeutics for several diseases. This review will present the recent progresses about the roles of NAD+-(ADP-R)n metabolism and the structures and functions of PARG, with a focus on its role in DNA repair and cell death by apoptosis in relation to central regulatory network, and the therapeutic potentials of PARG inhibitors in cancer chemotherapy.