Weighing functions are extremely useful tools for evaluating the relative effects of gases affecting global atmospheric ozone and climate. In particular, the concepts of Ozone Depletion Potentials (ODPs) and Global Warming Potentials (GWPs) are extensively used in policy consideration and scientific studies of the ozone and climate issues. ODPs provide a relative measure of the expected cumulative impact on stratospheric ozone from trace gas emissions, and are being used to examine the relative effects on ozone from CFCs, halons and other halocarbons currently being used and to evaluate the potential effects of possible replacement compounds. GWPs provide a means for comparing the relative effects on climate expected from various greenhouse gases. This chapter examines these weighing functions and other indices being used in evaluating concerns about global ozone and climate, discusses the science underlying these indices, and presents the current state-of-the-art for numerical indices and their uncertainties.