Infants' phonological acquisition during the first 18 months of life has been studied within experimental psychology for some 30 years. Current research themes include statistical learning mechanisms, early lexical development, and models of phonetic category perception. So far, linguistic theories have hardly been taken into account. These theories are based upon the assumption that there is a common core of innate phonological knowledge across speakers of all human languages, and they contain detailed proposals concerning phonological representations and the derivations by which abstract underlying forms are mapped onto concrete surface forms. It remains to be investigated experimentally if there is innate phonological knowledge and how the language-specific phonological grammar is acquired. In the present article, the contributions to this special issue are introduced, and an attempt is made to bridge the gap between phonological theory and experimental psychology. In particular, some recent experimental work is considered in the light of phonological theories and new research avenues are sketched. What might be innate, what needs to be acquired, and how this acquisition might take place are questions that are addressed with respect to several aspects of phonological knowledge, specifically segmental representations, phonotactics, phonological processes, and the architecture of the phonological grammar. (Contains 1 table and 11 footnotes.)