The concept of heteronuclear Long-Lived spin States (LLSs) is introduced. In the simplest case of a pair of heteronuclei, such states are given by the singlet order of the spin pair, which can be efficiently sustained under Zero or Ultra-Low Field (ZULF) conditions. Here we describe two possible ways of detecting long-lived singlet order of heteronuclei: detection at ZULF conditions and NMR (Nuclear Magnetic Resonance) detection at high field utilising fast field-cycling. A theoretical description of the underlying spin dynamics is presented for both cases; the discussion is supported by experimental examples of LLSs in 13CH groups. The generality of these phenomena is discussed, as well as their potential applications.