Requirements

Operating System

To install and run Bugzilla on your server, the core requirement is to have Perl installed. This means that Bugzilla can be installed on any platform where Perl can be installed, including Windows, Linux, and Mac OS X. Linux is highly recommended and is our 1st-class citizen operating system.

Hardware

The hardware requirements depend heavily on the size of the user base and on the number of bugs in your database. For small teams (up to a few tens of users) with few bugs (up to a few thousands bugs), any good machine will do the job pretty well. The important part is to have enough RAM (4 GB or more recommended) and a fast processor (for instance, 3 GHz). Such hardware is pretty common nowadays. A harddisk with 50 GB of free space is large enough.

Larger teams with several thousands of users and several hundreds of thousands bugs need more hardware. To give you an idea, the Bugzilla installation at Mozilla uses 8 servers: 3 webheads (12 GB of RAM each), 1 master database (35 GB of RAM), 3 slave databases (35 GB of RAM each), and 1 email relay. Moreover, mod_perl is enabled for improved performance. Note that it is designed for very high concurrent access, which is quite unusual, so your requirements are probably lower than that.

Software

Bugzilla needs a database server, a web server, and Perl. In all recommendations below, "Bugzilla requires version X" must be understood as "Bugzilla requires version X or newer". In all cases, the newer, the better, as newer releases have more bug fixes, are still supported and still get security fixes. If you install the mimimum version mentioned below, the risk is that this version will reach End Of Life pretty soon and so can be unsupported at any time.

Perl:
Bugzilla 4.4 and older require Perl 5.8.1 (or newer, see the previous paragraph), but Bugzilla 5.0 and newer will require Perl 5.10.1, and so we don't recommend Perl 5.8.x at this stage. Instead, you should install Perl 5.12 (though this version already reached End Of Life) or newer, as these newer versions have some useful improvements which will make your life easier.
Database Server:
Bugzilla supports MySQL, PostgreSQL, Oracle and SQLite. We highly recommend MySQL and PostgreSQL, which have the best support from Bugzilla and are used daily by Bugzilla developers. Oracle has several known issues and is a 2nd-class citizen. It should work decently in most cases, but may fail miserably in some cases too. SQLite is recommended for testing purposes only or for small teams.
If you install MySQL, version 5.0.15 is required by Bugzilla 4.x, but we highly recommend version 5.5 or newer. If you install PostgreSQL, version 8.3 is required.
Web Server:
Bugzilla has no minimum requirements for its web server. We recommend to install Apache 2.2, though Bugzilla works fine with IIS too (IIS 7 or higher recommended). If you install Apache and you want improved performances, you can enable its mod_perl module.