City-level greenhouse gas (GHG) emissions data is necessary as an input into identifying, planning, and monitoring urban climate change mitigation actions. Until recently, the availability of emissions data at the city level required the existence of a local emissions inventory, produced through painstaking local data collection. These inventories use different methodologies and urban boundary definitions, and are available for different years, making comparison or trend analysis across cities difficult. However, academic researchers and international organizations have used modeling techniques and proxy data from various sources to estimate city-level emissions globally. As a result, data sets are now available that estimate emissions, by sector, for thousands of cities worldwide.