Since the publication of the original white paper in 2008, Bitcoin and other blockchain-based currencies have soared in popularity and value---as of this writing, one Bitcoin is worth 54,000 USD. But what exactly is a Bitcoin? How can users make trustworthy transactions without a central banking authority? Why is Elon Musk always tweeting about $DOGE? We will answer most of these questions and explore the cryptographic methods and data structures underlying blockchains. I'll also discuss some active areas of related research.