In 2017 we have been hearing a lot about blockchain, in 2018 a bit less. But what is a blockchain, really? How is it connected to Bitcoin? Why is there the “crypto” in “cryptocurrencies”? And should we, as mathematicians, be even interested in it? We will be talking about these and similar questions during the seminar. We will briefly look at what kind of mathematics a usual blockchain is based on, how it started, and in what ways today’s world is trying to use it.