BACKGROUND: Over the past decade, there has been substantial improvement in outcomes after transcatheter aortic valve replacement. Many patient and procedural factors have also changed over that time, making it challenging to untangle the drivers of those improvements. METHODS: Among patients who underwent transcatheter aortic valve replacement from 2012 to 2018 within the Society of Thoracic Surgeons/American College of Cardiology Transcatheter Valve Therapies Registry, we examined the relative contribution of changes in patient factors, device modifications, improving experience/skill, and advances in periprocedural care to the observed improvement in outcomes after transcatheter aortic valve replacement. Mediator clusters included demographics, noncardiovascular comorbidities, cardiovascular comorbidities, device-related factors, and nondevice-related procedural factors. Using logistic regression, we serially adjusted for the mediator clusters to examine the contribution of each to the observed improvement in outcomes over time. RESULTS: Among 161 196 patients treated with transcatheter aortic valve replacement at 596 sites, outcomes improved steadily from 2012 to 2018, including 30-day mortality (6.7% to 2.4%), 30-day composite adverse events (25.3% to 10.5%), and 1-year mortality (19.9% to 10.1%; all <0.001). In sequential models, the unadjusted odds ratio for 30-day mortality was 0.82 per year (95% CI, 0.80-0.84), which was progressively attenuated with addition of each covariate cluster. Most of the improvement was explained by device factors and nondevice procedural factors. Results were similar for 30-day composite adverse events, although the observed temporal improvement was not fully explained by measured factors, suggesting improved technical skill as an additional mediator. In contrast to 30-day outcomes, each cluster of patient and procedural factors contributed similarly to the temporal improvement in 1-year mortality, indicating a greater impact of patient factors on longer-term outcomes. CONCLUSIONS: While US patients undergoing transcatheter aortic valve replacement have become younger, healthier, and lower risk over time, the most important factors contributing to improvements in short-term outcomes relate to advances in device technology and procedural factors, whereas changing patient characteristics had a greater impact on improvement in 1-year outcomes.