Millstone Grit is any of a number of coarse-grained sandstones of Carboniferous age which occur in the British Isles. The name derives from its use in earlier times as a source of millstones for use principally in watermills. Geologists refer to the whole suite of rocks that encompass the individual limestone beds and the intervening mudstones as the Millstone Grit Group. The term Millstone Grit Series was formerly used to refer to the rocks now included within the Millstone Grit Group together with the underlying Edale Shale Group.

The term gritstone describes any sandstone composed of coarse angular grains, and specifically refers to such sandstones within the Pennines and neighbouring areas of Northern England.

Rocks assigned to the Millstone Grit Group occur over a wide area of Northern England, where they are a hugely important landscape-forming element of the rock succession. They also occur in parts of northeast Wales and northwest Ireland. The group comprises a succession of sandstones, mudstones and siltstones, the specifics of the sequence varying from one area to another. They give rise both to a number of escarpments, known locally as edges, and a series of high plateaux throughout the region, many of which are of considerable cultural significance.

They are the major landscape-forming rocks of the northern part of the Peak District (the Dark Peak) and of its eastern and western flanks in the counties of Derbyshire, Staffordshire and Cheshire. The great expanses of moorland around Bleaklow and Black Hill and fringed with broken outcrops of gritstone are characteristic of the area.

The ‘eastern edges of the Peak District’ comprise a broadly north-south arranged series of west-facing cliffs from Bamford Edge in the north through Stanage Edge, Burbage Edge, Froggatt Edge, Curbar Edge, Baslow Edge, Gardom's Edge, Birchen Edge, Dobb/Chatsworth Edge, Harland Edge and Fallinge Edge in the south. To the east of these edges is a broad band of relatively flat moorland known as the Eastern Moors.

Towards the western margins of the Peak District are a rather more broken series of edges, facing in a variety of directions, from those surrounding the high plateaux of Kinder Scout and Combs Moss to the imposing crags of the Roaches, Hen Cloud and Ramshaw Rocks in the south.

A millstone shaped from Millstone Grit quarried in the area has been adopted as the emblem of the Peak District National Park. As an image, the millstone is widely visible on literature but use is made of the objects themselves at many of the entrances to the National Park.

These rocks extend northwards through the South Pennines of Lancashire and West Yorkshire and westwards into the Forest of Rossendale and West Pennines and the Forest of Bowland, also in Lancashire. At the Yorkshire Dales they cover the south-east edge, but north of the Craven Faults, due to weathering, they form only cappings to separate hills.