In Irish and Scottish mythology, The Cailleach (plural, Cailleachan) (also called The Cailleach Bheur) is generally seen as a divine hag, a creator, and possibly a deity. The word simply means 'old woman' in modern Scottish Gaelic, and has been applied to numerous mythological figures. In Scotland, she is credited with making numerous mountains and large hills, which are said to have been formed when she was striding across the land and accidentally dropped rocks from her apron.

In partnership with the goddess Brìde, the Cailleach is seen as a seasonal deity or spirit, with The Cailleach Bheur ruling the winter months between Samhuinn and Bealltainn, and Brìde ruling the summer months between Bealltainn and Samhuinn. Depending on local climate, the change in "rulership" is celebrated any time between Latha Fhèill Brìghde (February 1) at the earliest, Latha na Cailleach (March 25), or Bealltain at the latest.

In Scotland, the Cailleachan (lit. "old women") are also known as The Storm Hags, and seen as personifications of the elemental powers of nature, especially in a destructive aspect. They are said to be particularly active in raising the windstorms of spring, during the period known as A Chailleach.

One legend describes The Cailleach as turning to stone on Beltane and reverting back to humanoid form on Samhain in time to rule over the winter months. In Scotland, she ushers in winter by washing her plaid in the whirlpool of Coirebhreacain. When she is finished, her plaid is white and snow covers the land.

In Scotland and Ireland, the first farmer to finish the grain harvest made a corn dolly, representing The Cailleach, from the last sheaf of the crop. The last farmer to finish had the responsibility to take in and care for the corn dolly for the next year. Competition was fierce to avoid having to take in the Old Woman.

The word Cailleach (Scottish Gaelic) comes from a root meaning "veiled one", originally referring to nuns, and is related to "caileag" meaning "girl". The Lowland Scots word for "hag" is Carline which has evolved to mean witch.

The Cailleach Bheur has been described as having blue-black skin, like a corpse. In later tales she is the witch of Ben Cruichan, as recorded on tea-towels and postcards sold in the visitor shop for the Hollow Mountain. She is also credited with creating Loch Awe.

In art, she was depicted as a wizened crone with bear teeth and a boar's tusks. [citation needed]

She helped rescue Deò-ghrèine, along with Brian.