Anti-obesity medications used with lifestyle intervention produce greater and more sustained weight loss than does lifestyle intervention alone. However, until 2021, FDA-approved medications for the long-term treatment of obesity in the general adult population had not demonstrated the sustained loss of 15% body weight needed to meet or exceed all guideline-recommended targets for weight-related complications. To meet this need, investigators launched the Semaglutide Treatment Effect in People with obesity (STEP) program of phase 3 clinical trials to assess the safety and efficacy of a weekly 2.4-mg subcutaneous injection of semaglutide, a glucagon-like peptide-1 receptor agonist (GLP-1RA). Following the results of STEPs 1 to 4, the FDA approved semaglutide therapy in patients with obesity. This article examines the design, efficacy, and safety of semaglutide therapy as revealed by the results of STEPs 1 to 4. These trials included adults with obesity who reported at least 1 unsuccessful attempt to reduce body weight by means of diet. STEP 2 studied effects on patients who also had type 2 diabetes (T2D); STEPs 1, 3, and 4 excluded patients with this condition. STEP 3 examined the effect of this pharmacotherapy plus intensive behavioral therapy and a 2-step, intensively restricted dietary plan. STEP 4 assessed the effects of continuing semaglutide 2.4 mg vs switching to placebo after an initial treatment period. In the trials comparing the effects of semaglutide 2.4‑mg treatment with those of placebo across 68 weeks and in patients with no T2D (ie, STEPs 1 and 3), patients treated with semaglutide achieved a mean -14.9% to -16.0% weight change. This far exceeds the 4.0% to 10.9% weight loss seen with other approved antiobesity medications. In STEPs 2 and 4, the estimated treatment differences between the semaglutide 2.4-mg and placebo arms were -6.2% and -14.8%, respectively. Safety and tolerability of this treatment in STEPs 1 to 4 was consistent with those of other GLP-1RA-based therapies. Ultimately, the results of the first 4 STEP trials demonstrated that semaglutide 2.4 mg is a safe, well-tolerated, and highly effective treatment to promote weight loss, avoid weight regain, and mitigate the effects of the prevalent, chronic disease of obesity. In November 2022, based upon the results of STEPs 1 to 3 and other trials, the American Gastroenterological Association recommended that semaglutide 2.4 mg "be prioritized over other approved [anti-obesity medications] for the long-term treatment of obesity for most patients."