Background: Prescription drug misuse is second only to marijuana use as the nation's most common type of illicit drug use. As a result, understanding the prevalence of and reasons for prescription drug misuse has major public health implications. Policymakers can use this type of information to help inform their assessments of substance use prevention and treatment needs in their communities. Method: This report uses data from the 2015 National Survey on Drug Use and Health to provide up-to-date information on estimates of past year misuse of prescription drugs and reasons for the most recent prescription drugs misuse for adults aged 18 or older. Results: About 91.8 million adults aged 18 or older were past year users of prescription pain relievers in 2015, representing more than one-third (37.8 percent) of the adult population. About 11.5 million adults misused prescription pain relievers at least once in the past year, and the most commonly reported reason for their last misuse of a pain reliever was to relieve physical pain (63.4 percent). About 5.7 million adults misused prescription tranquilizers at least once in the past year. The most common reasons for misuse the last time were to relax or relieve tension (46.2 percent) and to help with sleep (21.2 percent). About 4.8 million adults misused prescription stimulants at least once in the past year. The most commonly reported main reasons for the misuse of stimulants among adults who misused stimulants in the past year were to help be alert or stay awake (28.4 percent), to help concentrate (26.2 percent), and to help study (22.4 percent). About 1.4 million adults misused prescription sedatives at least once in the past year; the most common reason for the last misuse was to help with sleep (73.2 percent). Conclusion: This study highlights that most people who misuse prescription drugs are doing so for the very reason that the substance is typically prescribed, and comparatively few were misusing the prescription drug because they were trying to get high. However, the misuse of many of these prescription drugs, such as prescription opioids, has documented risks, such as dependence, overdoses, and death.