Various observers agree that tourism contributes significantly to global greenhouse gas emissions. Under the Paris Agreement, countries around the world committed to reducing emissions to net zero by 2050, to limit global warming to well below 2¬°C. To avoid dangerous climate change, the tourism sector needs to decarbonize in line with this goal, which is challenging due to tourism's strong growth predictions. This chapter reviews existing literature and frameworks on how carbon emissions are measured in tourism across different scales (for an individual tourism business, a local destination, a country as well as for the aviation sub-sector, and at the global level), by providing examples of different approaches. I discuss the challenges that exist in assessing tourism's carbon footprint by taking note that these may vary between tourism sub-sectors. Also, I identify current knowledge gaps and pinpoint where the field of research needs to go from here.