Central banks are considering the issuance of a central bank digital currency to serve as a payment tool to support economic activities. A central bank digital currency can also serve secondary purposes that are related to the statutory objectives of a central bank. A central bank digital currency is digital fiat money and an important central bank innovation. It presents potential benefits and some risks which calls for cautious optimism about issuing central bank digital currencies and a need for central banks to think fast and slow about central bank digital currencies.