The river Wye rises on the eastern flank of Plinlimmon not far from the mountain's highest summit, and after a course of some 130 miles, south-easterly at first and southerly below Hereford, flows into the Severn estuary a little south of Chepstow.

Its source and the first ten miles of its course lie in Montgomeryshire, but a short distance below Llangurig, the first village on its banks, it enters Radnorshire, in which county it flows past Rhayader. From its junction with the Elan down to Hay the river forms the boundary between Radnorshire and Breconshire. The principal places situated on this part of its course are Newbridge-on-Wye, Builth Wells, Glasbury and Hay. For a short distance beyond Hay the Wye separates Breconshire from Herefordshire, but from Rhydspence (near Whitney) past Hereford and Ross to beyond Kerne Bridge it is entirely a Herefordshire stream.

The final and most famous portion of the Wye (Welsh Bicknor via Symonds Yat, Monmouth, Bigsweir Bridge and Tintern Abbey to Chepstow) forms (roughly speaking) the boundary between Monmouthshire and Gloucestershire, and Offa's Dyke runs along the Gloucestershire bank.

In the pages that follow every section of the Wye Valley is dealt with in turn, the upper portions being described as seen from their principal roads, the lower reaches as viewed from a boat on the river. Some account is given of every town of importance situated in the area, and a little space is also spared for the district of Llanidloes, which though outside the Wye basin is in touch with it, and for the spa-towns of Llandrindod Wells and Llanwrtyd Wells, which stand not on the Wye itself, but on tributaries. Special sections are devoted to Plinlimmon and the Source of the Wye ; to Hereford as a Touring Centre ; and to the Castles of the Lower Wye Valley.