The release came after 26 months of development. Debian releases are named after characters from the film Toy Story.
A project statement said the release was dedicated to Debian founder Ian Murdock who died on 28 December 2015.
The biggest change in Stretch is that the default MySQL variant will now be MariaDB, with the replacement of MySQL 5.5 or 5.6 packages being automatically replaced by the MariaDB 10.1 variant when a system is upgraded.
More than 90% of the source packages in Debian 9 will now build bit-for-bit identical binary packages, an important verification feature that will protect against attempts to tamper with compilers. In future, tools and metadata will be used so users can validate packages within the archive.
Stretch is the first Debian release to feature the "modern" branch of GnuPG which has elliptic curve cryptography, better defaults, a more modular architecture, and improved smartcard support.
Support for UEFI, which was first introduced in the last release Jessie, has been improved in Stretch, and now supports installing on 32-bit UEFI firmware with a 64-bit kernel. Debian live images now include support for UEFI booting as a new feature.
Debian has kept to its reputation for supporting the most architectures, with a total of 10 being supported. These are 64-bit PC / Intel EM64T / x86-64 (amd64), 32-bit PC / Intel IA-32 (i386), 64-bit little-endian Motorola/IBM PowerPC (ppc64el), 64-bit IBM S/390 (s390x), for ARM, armel and armhf for older and more recent 32-bit hardware, plus arm64 for the 64-bit "AArch64" architecture, and for MIPS, in addition to the two 32-bit mips (big-endian) and mipsel (little-endian), there is a new mips64el architecture for 64-bit little-endian hardware.
Support for 32-bit Motorola/IBM PowerPC (powerpc) has been removed in Stretch.
Debian can now be installed in 75 languages, most of which are available in both text-based and graphical user interfaces.