Joomla system requirements

In this article I've tried to gather system requiremets for different versions of Joomla CMS.

Joomla 2.5 System Requirements

PHP: 5.2.4 or above
MySQL: 5.0.4 or above
Apache: 2.x or above

Joomla 1.5 System Requirements

PHP: 4.3.10 or above (If you have PHP 5.3.x, you have to set error_reporting to E_ALL & ~ E_NOTICE)
MySQL: 3.23.x or above (but 6.x not supported)
Apache: 1.3.x or above

Joomla 1.0 System Requirements

PHP: 4.2.x or above (If you have PHP 5.3.x, you may read this article: Joomla 1.0 and PHP 5.3)
MySQL: 3.23.x or above
Apache: 1.13.19 or above

Source: Joomla Technical Requirements