Fentanyl is a potent mu opioid receptor agonist that was discovered to identify an improved human health analgesic over morphine, an opioid frequently associated with histamine-release, bradycardia, hyper- or hypotension, and prolonged postoperative respiratory depression. Historically, the pharmacological features of fentanyl have been described primarily through the study of the human approved fentanyl citrate formulation. In conscious dogs, fentanyl has a wide margin of safety, possesses minimum effects on the cardiovascular and respiratory systems, and is readily reversible. Other pharmacological features include sedation, mild reductions in body temperature, and dose-dependent reduction in food intake. The short duration of effect of available fentanyl citrate solutions has limited its clinical use to perioperative injections or constant rate infusions (CRIs). To extend the analgesic effect, additional fentanyl delivery technologies have been developed for human health including the fentanyl patch that has been used in an extra-label manner in dogs. Beyond the slow onset and variability in fentanyl delivery, several additional disadvantages have precluded common use of patches in dogs. The recent approval of long-acting transdermal fentanyl solution for dogs provides a new approach for sustained delivery of fentanyl for the control of postoperative pain in dogs. It has a rapid onset of action, prolonged duration, and mitigates the disadvantages of oral, parenteral, and patch-delivered opioids. The availability of a safe and effective approved opioid in dogs may allow further optimization of postoperative analgesia in this species. The objective of this review is to summarize the history and pharmacology of fentanyl and to integrate information about the newly approved long-acting transdermal fentanyl solution.