A backend engineer is responsible for the server-side application logic and integration of the work front-end engineers do. Backend engineers are usually write the web services and APIs used by front-end engineers and mobile application engineers. The application logic is calculated using a server-site programming language such as Java, C++, C#, Python, Javascript, etc which backend engineers must have a strong understanding of. They are also responsible for saving, updating, and reading data from the database which means they must have a decent amount of knowledge of databases.