In 1963, Chambon et al. reported the detection of a nicotinamide mononucleotide-activated, DNA-dependent enzymatic activity in rat liver extracts that catalyzed the synthesis of a polyadenylic acid. 1 The product of this reaction was later identified as poly (ADP-ribose) or PAR, a polymer of ADP-ribose (ADPR) monomers derived from the oxidized form of nicotinamide adenine dinucleotide (NAD+). 2 These initial studies have led to half a century of research on the chemistry, enzymology, structure, function, biology, physiology, and pathology of ADPR, PAR, and their derivatives, as well as the enzymes that catalyze their synthesis and degradation, and the effector proteins that interact with or are posttranslationally modified by them. In this Review, we describe the biological chemistry of PAR and its associated enzymes, effector proteins, and targets, with a particular emphasis on their roles in gene regulation, from chromatin to RNA biology.