ADP-ribosylation is a post-translational modification of proteins that occurs mostly in response to cellular stress and is catalysed by members of the diverse poly-ADP-ribose (PAR) polymerase (PARP/ARTD) family. The founding member of the family, PARP1, is best recognized for its function as a sensor of DNA strand lesions, but ADP-ribosylation has been implicated in transcriptional regulation, chromatin dynamics, telomere maintenance, apoptosis and neuronal signalling. Here we summarize a number of exciting recent breakthroughs in our understanding of the structural and mechanistic aspects of how PARP1 recognizes DNA, how PARPs are regulated, how ADP-ribose modifications are set onto specific targets and how the cellular machinery recognizes this elusive post-translational modification.