# What are Custom Controllers and Controller Extensions?

A custom controller is an Apex class that implements all of the logic for a page without leveraging a standard controller. Use custom controllers when you want your Visualforce page to run entirely in system mode, which does not enforce the permissions and field-level security of the current user.

A controller extension is an Apex class that extends the functionality of a standard or custom controller. Use controller extensions when:

- You want to leverage the built-in functionality of a standard controller but override one or more actions, such as edit, view, save, or delete.
- You want to add new actions.
- You want to build a Visualforce page that respects user permissions. Although a controller extension class executes in system mode, if a controller extension extends a standard controller, the logic from the standard controller does not execute in system mode. Instead, it executes in user mode, in which permissions, field-level security, and sharing rules of the current user apply.

:::note
Custom controllers and controller extension classes execute in system mode, so they ignore user permissions and field-level security. However, you can choose whether they respect a user's organization-wide defaults, role hierarchy, and sharing rules by using the `with sharing` keywords in the class definition. For information, see “Using the `with sharing`, `without sharing`, and `inherited sharing` Keywords” in the [Apex Developer Guide](https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_keywords_sharing.htm).
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