This disclosure pertains generally to prosthetic devices and related methods for helping to seal native heart valves and prevent or reduce regurgitation therethrough, as well as devices and related methods for implanting such prosthetic devices. In some cases, a spacer having a single anchor can be implanted within a native heart valve. In some cases, a spacer having dual anchors can be implanted within a native heart valve. In some cases, devices can be used to extend the effective length of a native heart valve leaflet.