The Natural Semantic Metalanguage (NSM) approach is well known for its use of reductive paraphrase as a mode of lexical definition and for its claim to have discovered an inventory of irreducible lexical meanings­semantic primes-that are apparently universal or near-universal in the world's languages (Wierzbicka 1996, 2006a, 2014; Goddard 201 la, 2012; Goddard and Wierzbicka 2002, 2014a, 2014b; Peeters 2006, 20 I 0, 2012; Levisen 2012; Ye in press; and other works). Sixty-five semantic primes have been identified. Examples include I, vou, SOMEONE, Do, KNOW, GOOD, BIG, BECAUSE. Semantic primes are analogous to atoms or elements of meaning. It is less well known that many NSM definitions (or explications, in NSM parlance) rely not only on semantic primes, but also on semantic molecules. This term refers to certain non-primitive meanings that function alongside semantic primes as integrated units ( or "building blocks") in the composition of yet more complex lexical meanings. It seems reasonable to expect that some semantic molecules are likely to be found in all or most languages. Examples include 'children,' 'long,' 'hands,' 'mouth,' 'sun,' 'water.' The focus of this paper is the NSM theory of semantic molecules. We begin however with a summary of key ideas about the NSM approach generally.