Map - Eilean Siar (Eilean Siar)

The Outer Hebrides or Western Isles (Na h-Eileanan Siar or Na h-Eileanan an Iar or Na h-Innse Gall ("islands of the strangers"); Waster Isles), sometimes known as the Long Isle/Long Island (An t-Eilean Fada), is an island chain off the west coast of mainland Scotland. The islands are geographically coextensive with Comhairle nan Eilean Siar, one of the 32 unitary council areas of Scotland. They form part of the archipelago of the Hebrides, separated from the Scottish mainland and from the Inner Hebrides by the waters of the Minch, the Little Minch, and the Sea of the Hebrides. Scottish Gaelic is the predominant spoken language, although in a few areas English speakers form a majority.

Most of the islands have a bedrock formed from ancient metamorphic rocks, and the climate is mild and oceanic. The 15 inhabited islands have a total population of and there are more than 50 substantial uninhabited islands. The distance from Barra Head to the Butt of Lewis is roughly 210 km.

There are various important prehistoric structures, many of which pre-date the first written references to the islands by Roman and Greek authors. The Western Isles became part of the Norse kingdom of the Suðreyjar, which lasted for over 400 years, until sovereignty over the Outer Hebrides was transferred to Scotland by the Treaty of Perth in 1266. Control of the islands was then held by clan chiefs, principal amongst whom were the MacLeods, MacDonalds, Mackenzies and MacNeils. The Highland Clearances of the 19th century had a devastating effect on many communities, and it is only in recent years that population levels have ceased to decline. Much of the land is now under local control, and commercial activity is based on tourism, crofting, fishing, and weaving.

Sea transport is crucial, and a variety of ferry services operate between the islands and to mainland Scotland. Modern navigation systems now minimise the dangers, but in the past the stormy seas have claimed many ships. Religion, music and sport are important aspects of local culture, and there are numerous designated conservation areas to protect the natural environment.

The earliest written references that have survived relating to the islands were made by Pliny the Elder in his Natural History, where he states that there are 30 Hebudes, and makes a separate reference to Dumna, which Watson (1926) concludes is unequivocally the Outer Hebrides. Writing about 80 years later, in 140–150 AD, Ptolemy, drawing on the earlier naval expeditions of Agricola, also distinguished between the Ebudes, of which he writes there were only five (and thus possibly meaning the Inner Hebrides) and Dumna. Dumna is cognate with the Early Celtic dumnos and means the "deep-sea isle". Pliny probably took his information from Pytheas of Massilia who visited Britain sometime between 322 and 285 BC. It is possible that Ptolemy did as well, as Agricola's information about the west coast of Scotland was of poor quality. Breeze also suggests that Dumna might be Lewis and Harris, the largest island of the Outer Hebrides although he conflates this single island with the name "Long Island". Watson (1926) states that the meaning of Ptolemy's Eboudai is unknown and that the root may be pre-Celtic. Murray (1966) claims that Ptolemy's Ebudae was originally derived from the Old Norse Havbredey, meaning "isles on the edge of the sea". This idea is often repeated but no firm evidence of this derivation has emerged.

Other early written references include the flight of the Nemed people from Ireland to Domon, which is mentioned in the 12th-century Lebor Gabála Érenn and a 13th-century poem concerning Raghnall mac Gofraidh, then the heir to the throne of Mann and the Isles, who is said to have "broken the gate of Magh Domhna". Magh Domhna means "the plain of Domhna (or Domon)", but the precise meaning of the text is not clear.

In Irish mythology the islands were the home of the Fomorians, described as "huge and ugly" and "ship men of the sea". They were pirates, extracting tribute from the coasts of Ireland and one of their kings was Indech mac Dé Domnand (i.e. Indech, son of the goddess Domnu, who ruled over the deep seas).