Posttranslational modifications (PTMs) regulate protein functions and 
interactions. ADP-ribosylation is a PTM, in which ADP-ribosyltransferases use 
nicotinamide adenine dinucleotide (NAD+) to modify target proteins with 
ADP-ribose. This modification can occur as mono- or poly-ADP-ribosylation. The 
latter involves the synthesis of long ADP-ribose chains that have specific 
properties due to the nature of the polymer. ADP-Ribosylation is reversed by 
hydrolases that cleave the glycosidic bonds either between ADP-ribose units or 
between the protein proximal ADP-ribose and a given amino acid side chain. Here 
we discuss the properties of the different enzymes associated with 
ADP-ribosylation and the consequences of this PTM on substrates. Furthermore, 
the different domains that interpret either mono- or poly-ADP-ribosylation and 
the implications for cellular processes are described.