In legal practice, “lives in being” describes the identifiable individuals who are alive at the date when a disposition (usually a trust, settlement or will) takes effect and whose lifetimes are used to measure the duration of contingent interests. It is central to the common law rule against perpetuities, under which certain future interests must vest, if at all, no later than 21 years after the death of the relevant lives in being.A life in being must be a human life, ascertainable when the interest is created (for example, the testator, named beneficiaries, or members of a defined class). The concept is used to test whether gifts, trusts, options or powers are void for remoteness.In England and Wales and Northern Ireland, modern perpetuity rules are largely statutory (for example, the Perpetuities and Accumulations Acts 1964 and 2009), which modify but do not abandon the underlying “life in being plus 21 years” idea. In Scotland and Ireland, the terminology is similar and the concept remains rooted in common law and statute, although detailed perpetuity reforms differ. Across all four jurisdictions, “lives in being” is a descriptive common law expression rather than a rigid statutory definition.