In this guest lecture, PhD candidate Stefan Driessen talks about blockchains, how they are built and when you would want to employ them in practice. We will look past the hype of cryptocurrencies and NFTs and talk about ledgers, decentralisation and consensus protocols. At the end of the lecture, we will discuss some state-of-the-art applications of blockchain technology; you are encouraged to come up with an application that you are curious about or think is over- or underrated.