William Hazledine (1763-1840) is a virtually forgotten figure in the history of engineering. He worked closely with Thomas Telford, and the list of their joint achievements in the UK is unmatched in the history of bridge engineering.These include the aqueducts of Chirk and Pontcysyllte; arch bridges at Bonar, Mythe, Tewkesbury and Craigellachie; lock gates and bridges for the Caledonian Canal and many smaller cast iron bridges; and, not least, the unprecedented suspension bridges at Conwy and Menai in North Wales. Hazledine's expertise also won him many other contracts, including the suspension bridge at Marlow in Buckinghamshire, and the ironwork for the first all-iron-framed multistorey building in the world, the Flax Mill at Ditherington in his home town of Shrewsbury. Recent research has begun to reveal his importance in the production of cast and wrought iron for these valued legacy structures. The paper highlights the outstanding quality of the iron that Hazledine made and the rigorous proof tests he undertook to verify the properties of the metal. These results will be of great interest to engineers today working on heritage castand wrought-iron structures, if only to learn of the high-quality results that were possible 200 years ago.