This book discusses literacy development in heritage language speakers and presents the results of four different quantitative studies that investigate the transfer of literacy skills in bi- and multilingual language development. The empirical studies focus on different populations of pupils, most of them located in various parts of Switzerland, and emphasise the potential residing in shared or transferred resources between their heritage languages and the languages spoken in the region to which their family has immigrated. The goal of all studies was to gain an understanding of the factors, both linguistic and non-linguistic in nature, that contribute to the development of language skills in both the heritage and school languages. Theoretical assumptions are put to the test via hypothesis testing and the generally shared assumptions on bilingual education are questioned based on the data. Furthermore, methodological problems in the investigation of linguistic interdependence are discussed. This book contributes to the scholarly investigation of potential beneficial effects in academic proficiency across languages in migrant children.