Digital currency (DC) has attracted strong attention in recent years. It has the potential to be widely used in payment and settlement. The governments and central banks around the world are closely monitoring the development of digital currencies. A number of countries (England, Sweden, Canada, India, China, Thailand, Philippines ...) are also conducting in-depth research on the central bank digital currency (CBDC) and its impact on operating the monetary policy, the financial system - banking and payment activities in the economy. An important policy question for the central banks is whether the digital currency should be issued by a central bank to be used in the payment system. This article will clearly provide definition as well as function of CBDCs that maybe helpful for policy markers in monitoring financial system in technology era. Besides, the paper also critically reviews the central bank's studies and evaluations in several countries to propose some suggestions.