Rationale: In patients with acute cerebral ischemia, the rate of stroke, myocardial infarction, or death during 90 days was reported to be non-significantly lower with ticagrelor compared with aspirin, with no increase in major hemorrhage. Dual antiplatelet therapy may be more effective in this setting. Aim: To investigate whether ticagrelor combined with aspirin are superior to aspirin alone in preventing stroke or death in patients with non-severe, non-cardioembolic ischemic stroke or high-risk transient ischemic attack. Design: The Acute Stroke or Transient Ischemic Attack Treated with Ticagrelor and Aspirin for Prevention of Stroke and Death (THALES) trial is a randomized, placebo-controlled, double-blind, event-driven study. Patients will be randomized within 24 h of onset of acute ischemic symptoms. THALES is expected to randomize 13,000 at ∼450 sites worldwide, to collect 764 primary outcome events. Study treatments are ticagrelor 180 mg loading dose on day 1, then 90 mg twice daily on days 2–30, or matching placebo. All patients will also receive open-label aspirin 300–325 mg on day 1, then 75–100 mg once daily on days 2–30. Study outcomes: The primary efficacy outcome is time to the composite endpoint of stroke or death through 30-day follow-up. The primary safety outcome is time to first severe bleeding event. Discussion: The THALES trial will provide important information about the benefits and risks of dual antiplatelet therapy with ticagrelor and aspirin in patients with acute cerebral ischemia in a global setting (funding: AstraZeneca). Clinical Trial Registration URL: http://www.clinicaltrials.gov. Unique identifier: NCT03354429. © 2019 World Stroke Organization.