This Special Issue (SI) is dedicated to early bilinguals, who acquire two languages during early childhood, before age 6, simultaneously (2L1 bilinguals) or sequentially (cL2 bilinguals). Recently, the notion of heritage speakers (HSs) − bilinguals who grow up speaking a minority language at home − has become prominent in this context. HS research has typically targeted bilinguals at a mature (adult) state, but early developing bilinguals may of course be HSs too, though not uniformly labelled as such. HSs of a moribund language or variety are another type of early bilingual, representing the final or penultimate, often 4th or 5th, generation of speakers. Unfortunately, ‘deficiency’ or ‘incompleteness’ is a common thread linking much HS research − despite a wealth of evidence demonstrating HSs’ maintenance of complexity in many grammatical domains along with differences to monolinguals (see, e.g., Kupisch & Rothman, 2016; Putnam & Sánchez, 2013). This SI brings together studies on how and why the morphosyntax and phonology of early bilinguals might differ from that of monolinguals.