Polygenetic volcanoes and calderas produce eruptions of a wide variety of magnitudes, chemistries, and recurrence times. Understanding the interplay between long- and short-term and deep and shallow processes associated with accumulation and transfer of eruptible magma is essential for assessing the potential for future eruptions to occur and estimating their magnitude, which remains one of the foremost challenges in the Earth sciences. We review literature and use existing data for emblematic volcanic systems to identify the essential data sets required to define the state of activity of volcanoes and their plumbing systems. We explore global eruptive records in combination with heat flux and other geological and geophysical data to determine the evolutionary stage of plumbing systems. We define a Volcanic Activity Index applicable to any volcano that provides an estimate of the potential of a system to erupt in the future, which is especially important for long-quiescent volcanoes. ▪ Magmatic plumbing systems that feed volcanic activity extend across Earth's crust and are long-lived at depth and ephemeral in their shallowest portions. ▪ We revise and update the definitions of active, quiescent, and extinct volcanoes based on physical proxies for the architecture, longevity, amount, and distribution of eruptible magma in the crust. ▪ We propose a Volcanic Activity Index, which provides a relative measure of the state of activity of a volcano with respect to all other volcanoes in the world. ▪ New imaging and monitoring strategies are required to improve our ability to detect lower and middle crust magmatic processes and forecast eruptions and their potential size. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 50 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates