This project aims to advance, theoretically and methodologically, our understanding of language development and outcomes in heritage language bilingualism (HLB). This research seeks to document and interpret the significance of differences in linguistic competencies and performances across individual heritage speakers (HSs) themselves. While typically HS aggregates are compared to homeland native speakers, it is at least of equal importance to unpack the degree and source of intra-group HS individual variation. Yet, this remains poorly investigated, much less understood. Limited research in this vein suggests that HS variation is largely modulated by individual-level factors, e.g., HL input, patterns of/opportunities for HL use, HL literacy and/or individual differences in cognitive abilities. However, individual/joint effects of these factors are not always observed depending on the specific linguistic (sub-)domains. This suggests an interaction between individual-level and language-internal factors. To understand this latter relationship more fully, HeLP examines Mandarin as a HL in pre-teenage (8-14) HSs, an understudied HL at an understudied time in HLB. Crucially, we include participants with varied language experience, testing narrow syntax of anaphoric dependencies, juxtaposed to contexts modulated by where they are moderated by discourse appropriateness. Rising to contemporary calls advocating for an expansion of HLB into more online methods, we employ electroencephalography, an effectively untapped method in HLB studies. As such, HeLP probes deeper into how HLs develop the way they do with neurolinguistic measures, which also bears social and economic impacts, e.g., guiding HSs and public and private sectors involved to better support HL development/maintenance.