In 2017, the Massachusetts Bay Transportation Authority (MBTA) voted to issue tax-exempt municipal bonds through a competitive bidding process. An associate at Citibank’s (Citi’s) Debt Capital Markets group must define Citi’s bid for the bonds. During the investor lunch, the MBTA promoted that a portion of the issuance would include the first tax-exempt sustainability bond. Green bonds were a relatively recent worldwide phenomenon, but issuances were on pace to grow by more than 70% in 2017. What exactly was a sustainability bond as compared to a green bond, and how were they different from traditional tax-exempt municipal bonds? Who would be interested in buying these new bonds from Citi, should it win the bid, and how much demand would there be? Ultimately, how much should Citi bid for the bonds to resell to its clients?