EPOC is a range of Operating Systems developed by Psion for portable devices, primarily PDAs. EPOC is rumored to be an acronym for "Electronic Piece Of Cheese."

Releases 1 and 2 (unofficially known as EPOC16) refer to operating systems used in Psion's "SIBO" devices, which have in common an 8086-family processor and a 16-bit architecture. SIBO devices include: MC200, MC400, Series 3, Series 3a, Series 3c, Series 3mx, Siena, Workabout and Workabout mx. The MC200, the first EPOC16 device, shipped in 1989, and as of November 2004, the Workabout mx (http://www.psionteklogix.com/public.aspx?s=com&p=Products&pCat=128&pID=1631) is still in production.

Releases 3, 4 and 5 of EPOC (unofficially known as EPOC32) are a brand new 32-bit operating system. Release 3 first appeared with the release of the Psion Series 5 in 1997. These were followed by the Series 5mx, Revo/Revo plus, Series 7/netBook and netPad (which all featured Release 5).

During development of EPOC32, Psion planned to license EPOC32 to third-party device manufacturers, and span off its software division as Psion Software. One of the first licensees was the short-lived Geofox, which halted production with less than 1,000 units sold. Ericsson marketed a rebranded Psion Series 5mx called the MC218, and later created an EPOC-based smartphone, the R380. Oregon Scientific also released a budget EPOC device, the Osaris (notable as the only EPOC device to ship with Release 4).

In June 1998, Psion Software became Symbian, a major joint venture between Psion and phone manufacturers Ericsson , Motorola and Nokia. As of Release 6, EPOC became known simply as Symbian OS.