Symbian is a proprietary operating system for wireless telecommunications, succeeding Psion Software’s EPOC. Released in 1999, it is owned by a number of companies: Nokia with 47.9% of the share, Ericsson with 15.6%, Sony Ericsson with 13.1%, Panasonic with 10.5%, Siemens AG with 8.4%, and Samsung with 4.5%.

Symbian was designed for handheld mobile devices by Symbian Ltd. It features associated libraries, interface frameworks and reference implementations of common tools. It is also capable of pre-emptive multitasking, multithreading and memory protection. Symbian operating system (OS) runs exclusively on Advanced RISC Machine (ARM; where "RISC" stands for "reduced instruction set computer") processors.

Symbian-specific programming idioms, such as descriptors and a clean-up stack, were devised specifically for conserving memory, a main focus of the Symbian OS since, being an operating system for handheld devices, it was developed with limited resources.

These programming idioms, along with a number of other techniques, minimize memory usage and make memory leaks an uncommon occurrence. Similar techniques have also been devised with the purpose of conserving disk space, although the disks on Symbian devices are usually flash memory.

A programming idiom called active objects is used to accomplish the task of switching off the central processing unit (CPU) when applications are not dealing directly with an event. This occurs due to the nature of Symbian OS programming being entirely event-based. Cumulatively, these techniques provide longer battery life.

The Symbian OS has a microkernel architecture, meaning that the minimum necessary is within the kernel, which contains a scheduler and memory management, but is without networking or filesystem support, which are functions provided by user-side servers.

The kernel is located at the lowest level of the Symbian OS, along with the other base components such as the user library. The user library allows user-side programs to make requests to the kernel. Also included in the base layer is the file server, which provides a view of the filesystems on the device in a manner similar to that of the Disk Operating System (DOS).

Numerous filesystem types such as File Allocation Table 32 (FAT32) and Symbian OS-specific NOR flash filing systems are all supported by Symbian OS. The phone user interface generally does not expose the filesystem to the user. System libraries are located at the level immediately above the base. At a level above this, the software is arranged into a stack.

Three main servers are a part of the large networking and communication subsystem of Symbian OS. These are EPOC telephony (ETEL), EPOC sockets (ESOC) and C32, and each of the three servers has a plug-in scheme.

Symbian OS devices may be programmed in a variety of languages such as C++, Simkin, Perl, Open Programming Language (OPL), Python, Visual Basic, Java Platform Micro Edition (Java ME), and PersonalJava.

The utilities offered by Symbian include the following: Internet Radio, PuTTY (terminal emulator application), Ruby Programming Language, SymTorrent, Symella, Python interpeter, Apache HTTP Server, Scumm VM (game emulator application) and OggPlay, which is an audio player that supports the ogg vorbis audio format.

As of 2006, Symbian OS is run over 100 million smart mobile devices. It has a 67% share of the smart mobile device market, followed by Microsoft with 15% and RIM with 6%.