Thyroid eye disease (TED) occurs quite commonly amongst patients with autoimmune thyroid disease but can also rarely appear clinically and radiologically in the euthyroid population. The natural history of the disease may vary between severity, ages and races, but in most cases follows the pattern of an ‘active’ inflammatory phase followed by an ‘inactive’ phase in which symptoms may persist due to fibrosis and residual tissue changes but does not progress further. It may be helpful to think of ‘Rundle’s curve’ as a guideline to the course of the untreated disease, but it is now understood that this may be an oversimplification. The disease course may be modified with reduction of peak severity by early intervention with disease modifying drugs. In this chapter, we will review the current knowledge of the course of TED.