ADP-ribosylation is a covalent posttranslational modification of proteins that is catalyzed by various types of ADP-ribosyltransferase (ART) enzymes, including members of the poly(ADP-ribose) polymerase (PARP) family. ADP-ribose (ADPR) modifications can occur as mono(ADP-ribosyl)ation, oligo(ADP-ribosyl)ation, or poly(ADP-ribosyl)ation, depending on the particular ART enzyme catalyzing the reaction, as well as the specific reaction conditions. Understanding the biology of ADP-ribosylation requires facile and robust means of generating and detecting the modification in all of its forms. Here we describe how to generate protein-linked mono(ADP-ribose), oligo(ADP-ribose), and poly(ADP-ribose) (MAR, OAR, and PAR, respectively) in vitro as an automodification of PARPs 1 or 3. First, epitope-tagged PARP-1 (a PARP polyenzyme) and PARP-3 (a PARP monoenzyme) are expressed individually in insect cells using baculovirus expression vectors, and purified using immunoaffinity chromatography. Second, the purified recombinant PARPs are incubated individually in the presence of different concentrations of NAD+ (as a donor of ADPR groups) and sheared DNA (to activate their catalytic activities) resulting in various forms of auto-ADP-ribosylation. Third, the products are confirmed using ADPR detection reagents that can distinguish among MAR, OAR, and PAR. Finally, if desired, the OAR and PAR can be deproteinized. The protein-linked and free MAR, OAR, and PAR generated in these reactions can be used as standards, substrates, or binding partners in a variety of ADPR-related assays.