Dysthyroid optic neuropathy (DON) is a rare but feared complication of thyroid eye disease (TED), in which optic nerve function becomes impaired due to apical compression and microvascular compromise. Persistent DON can lead to permanent sight loss. Approximately 5% of people with TED will progress to experience DON, and early accurate diagnosis is essential to allow prompt treatment of those at risk of loss of vision. Treatment is usually with intravenous corticosteroid plus or minus surgical decompression. This chapter will outline the natural history of the condition and its management.