Databases are a crucial part of every web application as they are the place where all information processing happens. To choose the right database for an application, it is necessary to look closely at the use case and the data to store. This chapter will firstly illustrate in detail which functions have to be fulfilled by a web application database. It will then highlight the main features of SQL databases and will shortly present MySQL, PostgreSQL and MariaDB as examples hereof. It will continue by outlining the relevance of NoSQL databases for web applications, introducing Redis, MongoDB and Cassandra as such. Finally, the chapter will conclude with practical tips on how to choose a suitable database for a web application.