Blockchain is an emerging technology. Thanks to its immutability, transparency, the consensus mechanism for avoiding double spending, along with other clever designs such as blocks chained with the hashes of the previous blocks, the technology allows untrusting parties to trade with each other. In this chapter, we explained the basic concepts of its important features. Most of the discussions were about Bitcoin, which is the mother of the technology. We briefly talked about Ethereum, which extended Bitcoin and introduced the concept of smart contracts. The introduction of smart contracts makes the Ethereum blockchain generic and allows us to develop applications beyond the borderless cash payment use case for which Bitcoin was invented. The concept of an enterprise chain, along with one of the examples, Hyperledger, was mentioned as well. Finally, we briefly touched on the evolution of blockchain to give readers an idea of the trend in the technology. In the next chapter, we will discuss the concepts of Ethereum in detail.