Jonathan Gruber teaches this session of MIT's 14.41, Public Finance and Public Policy, on cost benefit analysis as a tool for evaluating government decisions. He works through how to measure the benefits of a policy against its costs, including how to value non-market goods like time and human life, how to discount future costs and benefits to present value, and how to handle uncertainty in the estimates. Gruber uses concrete policy examples to show where cost benefit analysis breaks down in practice, such as disagreements over the discount rate or the value assigned to a statistical life. The 77 minute lecture is chalk-and-talk style, working through the logic step by step with student questions along the way, and fits into the course's broader treatment of when and how government should intervene in the economy.