Quantifying greenhouse gas sources and sinks is essential for assessing the success of emission reduction efforts. Observations of atmospheric greenhouse gases have become major tools for tracking these fluxes. Because atmospheric transport links emissions to downwind concentration changes, this presents an inverse problem. In this presentation, I’ll introduce the general methodology for quantifying emissions, highlighting key successes and ongoing challenges. I’ll also provide an overview of diverse applications, from tracking emissions at individual facilities to monitoring trends on a global scale.