In order to really get a grasp on Drupal, especially how Drupal models data, it's necessary to have at least a familiarity with the Entity API. We start off with an overview of the terminology associated with the Entity system. We then look at a high level to see how the various components come together to form an API. Our configuration management series already has tutorials on Configuration Entities, but in this series we introduce how to construct your own custom content entity. We will also take time to learn how entities can be created, read, updated and deleted programmatically from elsewhere in our code as well as how to use the entity query service to identify entities matching particular conditions.