Transcatheter aortic valve implantation (TAVI) is a key-hole technique to replace an aortic heart valve that is narrowed and/or leaking. Although TAVI is a safe and effective treatment for a faulty aortic heart valve, the new TAVI valve will not last forever. Because it is a 'tissue' valve (made from the lining of a cow or pig heart), the valve will fail after a period of time as the tissue degenerates. When the TAVI valve fails, a viable treatment option is to perform a 'Redo TAVI' procedure, implanting a second TAVI valve inside the first failing valve. The main purpose of this study is to carefully evaluate patients being treated by Redo TAVI in order to document the short-term and long-term outcomes of the procedure. The study will also obtain information about which factors predict those outcomes. The study will also assess outcomes in patients who present with TAVI valve failure but are not suitable for Redo TAVI, and instead are treated either by open-heart surgery and surgical aortic valve replacement, or by medical therapy (medication). The study will provide doctors the information they need to understand the best way to treat patients who present with TAVI valve failure, and in particular how to perform Redo TAVI procedures with the best possible outcomes for patients.