Plone is an extensible content management system written in the Python programming language. It is based on Zope. Plone is free software and is designed to be extensible. It can be used as an intranet or extranet server, a document publishing system, and a groupware tool for collaboration between separately located entities.

The Plone project was started in 1999 by Alan Runyan, Alexander Limi, and Vidar Andersen. It has quickly grown into one of the most popular and powerful open-source content management systems in the world. In 2004, the Plone Foundation was formed to protect and promote the use of Plone.

Plone is released under the GNU General Public License. It is built on top of the open-source application server Zope and the accompanying Zope Content Management Framework, which has thousands of developers around the world supporting it. Plone development is often furthered by periodic Plone Sprints. Plone is customized using Products, which are distributed both through the Plone Web site, and through a variety of independent Web sites. The Plone collective contains a number of the more popular Plone Products.