Web server
Apache, Nginx, or Microsoft IIS
Database
Drupal 8:
MySQL 5.5.3/MariaDB 5.5.20/Percona Server 5.5.8 or higher with PDO and an InnoDB-compatible primary storage engine,
PostgreSQL 9.1.2 or higher with PDO,
SQLite 3.6.8 or higher
Drupal 7:
MySQL 5.0.15/MariaDB 5.1.44/Percona Server 5.1.70 or higher with PDO,
PostgreSQL 8.3 or higher with PDO,
SQLite 3.3.7 or higher
Drupal 6:
MySQL 4.1.1 or higher,
PostgreSQL 7.1
Microsoft SQL Server and Oracle are supported by additional modules.
PHP
Drupal 8: PHP 5.5.9 or higher
Drupal 7: PHP 5.2.5 or higher (5.4 or higher recommended).
Drupal 6: PHP 5.x only (5.2.5 or higher recommended). Warning: support for PHP 4.x has been dropped. Drupal core should work with PHP 5.3.x, but PHP 5.3.x and higher may produce errors or unexpected behavior especially for contributed modules and themes.