This article explores theoretical conditions necessary for “quantum immortality” (QI) as well as its possible practical implications. It is demonstrated that the QI is a particular case of “multiverse immortality” (MI) which is based on two main assumptions: the very large size of the Universe (not necessary because of quantum effects), and the copy-friendly theory of personal identity. It is shown that a popular objection about the lowering of the world-share (measure) of an observer in the case of QI doesn’t (...) work, as the world-share decline could be compensated by the merging timelines for the simpler minds, and also some types of personal preferences are not dependent on such changes. Despite large uncertainty about MI’s validity, it still has appreciable practical consequences in some important outcomes like suicide and aging. The article demonstrates that MI could be used to significantly increase the expected subjective probability of success of risky life extension technologies, like cryonics, but makes euthanasia impractical, because of the risks of eternal suffering. Euthanasia should be replaced with cryothanasia, i.e. cryopreservation after voluntary death. Another possible application of MI is as a last chance to survive a global catastrophe. MI could be considered a plan D of reaching immortality, where plan A consists of the survival until the creation of the beneficial AI via fighting aging, plan B is cryonics, and plan C is digital immortality. (shrink)