Carbon budgets are becoming an integral part of climate policy. What are carbon budgets? Where did they come from? How are they linked to climate targets? These are some of the questions addressed in this chapter. Since the work of IPCC AR5 WG1 temperature limits, which have long been used as short hand for the impacts of climate change, now come with an implicit carbon budget, that is the cumulative anthropogenic CO2 emissions that can be released to the atmosphere while remaining below a given temperature limit. Keeping within the carbon budget will keep climate in a ‘safe operating space’ for humans.