A blockchain is a distributed network which functions as a digit ledger and a smart contract allowing the secure transfer of assets without an intermediary. Bitcoin, a P2P electronic cash system, is the first manifestation of the blockchain technology. As the internet is a technology to facilitate the digit flow of information, the blockchain is a technology to facilitate the digital exchange of value. Due to its distributed and secure nature, blockchain technology is believed to be the next generation digital exchange platform with wide applications such as cryptocurrency, healthbank...etc. While the blockchain technology is conceptually powerful, it suffers from two major problems: scalability and security. In this talk, I will first give a "crash course" on the blockchain protocols, e.g. PoW (Proof of Work) and PoS (Proof of Stake). I will then focus on the PoS model, and show how this design few challenges and research directions in the blockchain designs which are worth further developments. The work is based on joint work with David D. Yao.