This Phase 3 study evaluates the efficacy and safety of once-weekly subcutaneous tirzepatide (a dual GIP and GLP-1 receptor agonist) at 5 mg, 10 mg, or 15 mg versus placebo, each used with a reduced-calorie diet and increased physical activity, in adults without type 2 diabetes who have obesity or are overweight with weight-related comorbidities. The primary efficacy assessment is at Week 72. Participants with prediabetes at randomization may continue in an additional long-term treatment period to assess progression to type 2 diabetes and longer-term weight outcomes.