All human languages have simple and complex words. Simple words refer to meanings regardless of their form, while complex words are formed from other words, and their formation can be semantically motivated. Since words can share lexical material, we can group them into families. Word families can vary greatly in size, ranging from small ones – comprising only a few members –, to large ones – spanning several hundred words –, but it is still unclear why some words are more productive than others in forming new words. Lexical compositionality has received some attention in historical linguistics, linguistic typology, and cognitive linguistics, but so far studies have mostly concentrated on the morphological complexity of individual words and languages, while the fact that words form families which interact during language change and language use has been typically ignored. As a result, many questions regarding word family formation remain unresolved, and we do not know 1) how word families evolve along language phylogenies, 2) which semantic processes underlying word family formation are universal, and 3) to what extent human cognition influences the productivity of lexical roots to form families. The focus stream invites contributions that tackle the three target questions or additional questions related to the evolution, the typology, or the psychology of word families and lexical compositionality.