De Quervain's tenosynovitis (DQT) is a repetitive stress condition located at the first dorsal compartment of the wrist at the radial styloid. The extensor pollicis brevis (EPB) and abductor pollicis longus (APL) tendons and each tendon sheath are inflamed and this may result in thickening of the first dorsal extensor sheath. Workers who perform repetitive activities of the wrist and hand and those who routinely use their thumbs in grasping and pinching motions in a repetitive manner are most susceptible to DQT. Conservative treatments include activity modification, modalities, orthotics, and manual therapy. This chapter identifies, in an evidence base manner through the literature, the most effective diagnostic measures for DQT. It also examines the evidence base on (or lack thereof) the treatment or treatment combinations to reduce pain and improve functional outcomes for patients with DQT.