Blockchain technology has been one of the fastest growing technologies with wide applications in the past decade. However, major blockchains, such as bitcoin and Ethereum, suffer low transaction rates and high transaction processing latencies, which hinder blockchains’ scalability. The layer-1 scalability solutions have their own limitations. Layer-2 scalability solutions are widely considered to be the most important approach to solving the blockchain scalability issues. There already exist several methods: payment state channels, plasma, rollups, validium, etc. This reading course will cover the most recent advances in this topic, particularly on zk rollups. Layer-2 blockchain protocols have the potential to the game changer in the blockchain industry. Students should seek the course instructor’s approval to take this course.