The Pennine system is often wrongly called a chain, but it is hardly even a range. Its hills are broken up into numerous short ranges by valleys (often called dales) cut back into them in every direction. The Pennines, in fact, form a north and south watershed that determines the course of all the larger rivers in northern England. The Pennines are divided into two main sections by a gap formed by the Rivers Aire (flowing east) and Ribble (flowing west). The northern section of the Pennines is broader and generally higher than the southern. The highest points in the northern section are Cross Fell (2,930 feet 【893 m】), Whernside (2,419 feet 【737 m】), Ingleborough (2,373 feet 【723 m】), and Pen-y-Ghent (2,273 feet 【693 m】). In the southern section, heights of more than 2,000 feet (600 m) are rare, apart from Kinder Scout (2,088 feet 【636 m】), part of the Peak District of Derbyshire.

The geological structure of the Pennines consists of carboniferous limestone and Millstone Grit with some local shales. On the drier areas, heather moor predominates, while the wet, peaty areas are covered mostly with cotton grass. The summits of the hills are rounded or nearly flat, but geological structures and glacial action have helped to produce fine scenery in the dales. Water action has developed remarkable underground caverns and watercourses in the limestone of the Pennines. Among these caverns and chasms are Ingleborough Cave near Clapham, Gaping Gill (more than 350 feet 【107 m】 deep), and Rowten Pot (365 feet 【111 m】). The stream draining Malham Tarn (brook) disappears below ground and reappears at the foot of the cliffs at Malham Cove. A notable underground watercourse in Derbyshire is the River Wye, which disappears into Plunge Hole and then traverses Poole's Hole, near Buxton. There are few lakes in the Pennines, but reservoirs in Millstone Grit areas supply the manufacturing regions of West Yorkshire and Lancashire with water.

The economy of the Pennines is based mainly on sheep farming and the quarrying of limestone. The valleys contain numerous small market towns, among them Hawes, Muker, and Grassington. Tourism has become an important element in the economy, helped by the designation of the Peak District, Yorkshire Dales, and Northumberland national parks. The Pennine Way, a footpath running along the hills of the Pennines from end to end for 250 miles (400 km), was opened in 1965.

There are numerous prehistoric remains, such as the great circle of stones at Arbor Low Hill. Hadrian's Wall, an ancient Roman defensive line against the peoples of what was, in large part, to become Scotland, extends east-west along the northern edge of the Pennines.