Because of their current value, cryptocurrencies are one of the more popular topics in computer society. To most of the people, they are just the way of paying, but can be so much more and this master thesis focuses on researching cryptocurrencies in that wider context. It presents them as a decentralized network and gives examples in which way they affect everyday life. Smart contracts give us a possibility to write an arbitrary computer program, send it in a transaction and then run it on the network. A decentralized application is a name for modern application build from smart contracts. A purpose of this thesis is full implementation of one application of that kind and researching theoretic background to understand its working. Our decentralized application is called Crypto Roulette and hosts on: https://masterthesisv2.firebaseapp.com/. In the first part of theses initially, we state all theoretical knowledge required to understand the decentralized application. In the beginning, we present cryptography that makes a base for all cryptocurrencies so later we can introduce Bitcoin, and after that Ethereum - which is a network where our smart contracts runs. After that we will observe contracts itself, their possibilities, running prices, security and ways to call code. The second part focuses on the practical use of theory from the first part when building a decentralized application. Therefore we present the architecture of the entire application while listing tools used in the development process. Finally, we present an application itself, analyze code of the Crypto Roulette smart contract and emphasize differences between ”classical” application development and decentralized one. In the conclusion part, we list examples from every day where smart contracts are used and our predictions for the future.