This project aims to fill important gaps in our existing knowledge of Heritage Language (HL) acquisition, a subfield of bilingualism studies. While most previous research has focused on describing how HLs diverge from their monolingual baselines, the main objective of HeLA is to understand why heritage grammars develop the way they do. To this end, we focus on the severely understudied population of pre-teenage heritage speakers (HSs) of Spanish. While much attention has been paid to the role of input quantity in HL acquisition, relatively little is known about the qualitative nature of that input. This project investigates input quality by focusing on cross-generational attrition: the situation where HSs’ parents might exhibit changes in their L1 use –due to prolonged exposure to a second language and lack of activation in their first language– which get passed on to the next generation of HSs. We will approach this topic in a unique and novel way, namely by including individual analyses of children and their respective parents, and establishing a one-to-one connection between the two. Thus far, the majority of research on heritage Spanish, has been carried out in the context of the US. HeLA offers a valuable contribution to the field by comparing Spanish in two different linguistic contexts in Europe: the UK and The Netherlands. By looking at phenomena that are instantiated differently in English and Dutch –grammatical gender and subject expression– we will gain more insight into the role of cross-linguistic influence from the majority language. We furthermore combine behavioural and online measures of competence to examine whether the type of task brings anything to bear on the participants’ performances. Gaining a deep understanding of the factors that shape heritage grammars is of considerable importance for the maintenance of immigrant languages and cultures, as well as for stakeholders in the public and private sector charged with supporting such efforts.