Peer-to-peer (P2P) networking is a distributed application architecture built upon the principle of the equality of participants. Commonly, P2P networks do not have dedicated servers and every peer takes a part of the work. P2P systems have been used in many application domains but became popular after the launch of the P2P file sharing system called Napster. Today, P2P networks are known for being platforms for the development of cryptocurrencies, such as Bitcoin, Ether and many others. P2P networks have proven themselves as robust, useful and secure, which then led to the establishment of P2P payment systems and to their growth in popularity. Current P2P networks are used in finance, content delivery, peer-to-peer file sharing, copyright infringement, multimedia and distributed search engines for building secure and trusted networks.