This lecture discusses the concept of marginal abatement costs and their significance in reducing emissions. It begins with an overview of the marginal abatement cost distribution, illustrating how different measures to reduce emissions can be represented in a histogram format. The instructor explains the importance of implementing the cheapest measures first, emphasizing the economic rationale behind this approach. The discussion then shifts to the implications of pricing emissions, including the effects of taxes and subsidies on emission reductions. The instructor elaborates on how emitters compare the costs of paying for emissions versus the costs of implementing reduction measures. Various mechanisms for setting prices on emissions, such as taxes and cap-and-trade systems, are explored, highlighting their respective advantages and challenges. The lecture concludes with a discussion on the impact of exemptions for certain emitters and how this can affect overall emission reduction targets, stressing the need for equitable policies that encourage participation from all sectors.