Mac OS 9 was the final major release of Apple's "Classic" Mac OS. Introduced on October 23, 1999, Apple positioned it as "The Best Internet Operating System Ever," highlighting Sherlock 2's Internet search capabilities, integration with Apple's free online services known as iTools, and improved Open Transport networking.

While Mac OS 9 lacks the functionality of a modern operating system, such as protected memory and full pre-emptive multitasking[1], lasting improvements include the introduction of an automated Software Update engine and support for multiple users.

Apple discontinued development of Mac OS 9 in 2002, transitioning all future development to Mac OS X. Since that time, no updates have been released. The final updates to Mac OS 9 addressed compatibility issues with OS X while running in the Classic Environment and compatibility with Carbon applications.

Apple billed Mac OS 9 as including "50 New Features" and heavily marketed its Sherlock 2 software, which introduced a 'channels' feature for searching different online resources and introduced a QuickTime-like metallic appearance. Mac OS 9 also featured integrated support for Apple ’s suite of Internet services known as iTools (later re-branded as .Mac) and included improved TCP/IP functionality with Open Transport 2.5.

Other features new to Mac OS 9 include:

Integrated support for multiple user accounts without using At Ease.
Support for voice login through VoicePrint passwords.
Keychain, a feature allowing users to save passwords in protected keychains.
A Software Update control panel for automatic download and installation of Apple system software updates.
A redesigned Sound control panel and support for USB audio.
Speakable Items 2.0, also known as PlainTalk, featuring improved speech synthesis and recognition along with AppleScript integration.[4]
Improved font management through FontSync.
Remote Access Personal Server 3.5, including support for TCP/IP clients over PPP.
An updated version of AppleScript with support for TCP/IP.
Personal File Sharing over TCP/IP.
USB Printer Sharing, a control panel allowing certain USB printers to be shared across a TCP/IP network.
128-bit file encryption in the Finder.
Support for files larger than 2 GB.
Unix volume support.
CD Burning in the Finder (introduced in Mac OS 9.1).
Addition of a 'Window' menu to the Finder (introduced in Mac OS 9.1)