Blockchain technologies are beginning to mature and are triggering great interest both in academic and industrial fields. At the technical level, the Blockchain
incorporates concepts from different areas of knowledge such as computer networks, distributed systems, consensus algorithms, security and cryptography,
programming and game theory to create a state machine equally shared among many entities. In this talk I will start by describing a classical centralized
electronic coin system like e-cash, then,I will discuss consensus in permissioned environments and finally, I will show how consensus can be achieved in open systems like public Blockchains.
At the end of the talk, I will also discuss how the two main blockchain systems (Bitcoin and Ethereum) manage their state and some applications.