It runs north from the A35 at Dorchester in Dorset into Somerset through Yeovil and Shepton Mallet before terminating at the Three Lamps junction with the A4 in central Bristol.

The road is entirely single carriageway, except in the Yeovil and Bristol built-up areas, at Ilchester (where it multiplexes with the A303), and north of Dorchester.

The original A37 in 1922 started in Fortuneswell, Isle of Portland, however, the section to Dorchester was soon renumbered A354, presumably to create a link between the major port of Weymouth and the A30 at Salisbury, from where the route would continue to London.

After a long campaign for the junction to be replaced, a roundabout was constructed in 1999 significantly improving road safety and traffic flow.

In recent years most of the overtaking lanes, provided on steep stretches just south of Bristol, have been blocked out with chevrons.