TYPO3 CMS is entirely built around the concept of extensions. The Core itself - since version 7 - is entirely comprised of extensions, called “system extensions”. Some are required and will always be activated. Others can be activated or deactivated at will.

Many more extensions - developed by the community - are available in the TYPO3 Extension Repository (TER).

Yet more extensions are not officially published and are available straight from source code repositories like GitHub.

It is also possible to set up TYPO3 CMS using Composer. This opens the possibility of including any library published on Packagist.

The architecture of an extension is described in detail in the Core APIs Reference.

Package Manager

Extensions are managed by a Package Manager. The two most visible parts of this tool are the \TYPO3\CMS\Core\Package\PackageManager PHP class and the typo3conf/PackageStates.php file.

The typo3conf/PackageStates.php file contains a list of all active packages.

Here is an extract of this file:

# PackageStates.php

# This file is maintained by TYPO3's package management. Although you can edit it
# manually, you should rather use the extension manager for maintaining packages.
# This file will be regenerated automatically if it doesn't exist. Deleting this file
# should, however, never become necessary if you use the package commands.

return [
        'packages' => [
                'core' => [
                        'packagePath' => 'typo3/sysext/core/',
                'extbase' => [
                        'packagePath' => 'typo3/sysext/extbase/',
                'fluid' => [
                        'packagePath' => 'typo3/sysext/fluid/',
                'install' => [
                        'packagePath' => 'typo3/sysext/install/',
                'frontend' => [
                        'packagePath' => 'typo3/sysext/frontend/',
                // ...
        'version' => 5,


You should not edit this file yourself, unless you know exactly what you are doing.

Main system extensions

This section describes the main system extensions, their use and what main resources and libraries they contain. The system extensions are located in directory typo3/sysext.


As its name implies, this extension is crucial to the working of TYPO3 CMS. It defines the main database tables (BE users, BE groups, pages and all the “sys_*” tables. It also contains the default global configuration (in typo3/sysext/core/Configuration/DefaultConfiguration.php). Last but not least, it delivers a huge number of base PHP classes, far too many to describe here.


A nice way to browse TYPO3 CMS Core classes is to use http://api.typo3.org/.


This system extension provides all that is necessary to run the TYPO3 CMS backend. This means quite a few PHP classes, but also a lot of Fluid templates.


Most backend labels reside in system extension “lang”.

This system extension contains all the tools for performing rendering in the frontend, i.e. the actual web site. It is mostly comprised of PHP classes, in particular those in typo3/sysext/frontend/Classes/ContentObject, which are use for rendering the various content objects (one class per object type, plus a number of base and utility classes).
Extbase is a MVC framework (with the “View” part being actually system extension “fluid”) originally derived from the Flow framework. Not all of the TYPO3 CMS backend is written in Extbase, but many modules are, with more being migrated to that framework with each new version.
Fluid is a templating engine. It forms the “View” part of the MVC framework. The templating engine itself is provided as “fluid standalone” which can be used in other frameworks or as a standalone templating engine. This system extension provides a number of base classes and many View Helpers (in typo3/sysext/fluid/Classes/ViewHelpers), which extend the basic templating features of Fluid. Fluid can be used in conjunction with extbase (where it is the default template engine) or standalone.
This system extension is the package containing the TYPO3 CMS Install Tool.