# Back-end customization
🤓 What is Koa?
If you are not familiar with the Koa back end framework, we highly recommend you to read the Koa's documentation introduction (opens new window).
Each part of Strapi's back end can be customized:
the requests received by the Strapi server,
the routes that handle the requests and trigger the execution of their controller handlers,
the policies that can block access to a route,
the middlewares that can control the request flow and the request before moving forward,
the controllers that execute code once a route has been reached,
the services that are used to build custom logic reusable by controllers,
the models that are a representation of the content data structure,
the responses sent to the application that sent the request,
and the webhooks that are used to notify other applications of events that occurred.