Most applications that you will work with or encounter will involve the use of some sort of data storage container. In some cases, you will need nothing more than files for this purpose, but often, that container is some sort of database engine. PHP provides access to a great number of different database systems, many of which are relational in nature and can be interrogated using Structured Query Language (SQL). In order to utilize these databases, it is important to have a firm grasp of SQL, as well as the means to connect to and interact with databases from PHP. This chapter reviews the basic concepts of SQL and database connectivity from PHP using PHP Data Objects (PDO).