Poly(ADP-ribosyl)ation of proteins is a posttranslational modification mediated by poly(ADP-ribose) polymerases (PARPs) that use NAD+ as substrate to form the negatively charged polymer of poly(ADP-ribose) (PAR). After DNA damage, PARP-1 is responsible for approximately 90% of the total cellular PARylation activity. Numerous studies showed activation of PARP-1 in various conditions associated with oxidative and nitrosative stress, such as ischemia-reperfusion injury, diabetes mellitus, and inflammation, and also proved the beneficial effects of PARP inhibitors. Several pharmacological inhibitors of PARP moved toward clinical testing for a variety of indications, including cardioprotection and malignant tumors, and in late 2014, olaparib became the first PARP inhibitor approved for human use for the therapy of ovarian cancer. These advances necessitate the detection of PARP activation in human tissues. In the present chapter, we review specific methods used to detect PARP activation in human circulating leukocytes and in human tissue sections.