The complicated task of deciding whether reading and writing difficulties in a second language learner stem from dyslexia or from problems associated with second language learning serves as the primary theme of this study. The theoretical framwork of dyslexia-related issues is a phonological cause model (Ramus, 2004). Generally, the study is based on psycholinguistically oriented research of reading in a second language (eg. Kulbrandstad, 1998) and dyslexia in second language learners (Frederickson & Frith, 1998; Geva, 2000; Miller Guron & Lundberg, 2003). The overall aim of the study is to contribute to our knowledge of how reading and writing difficulties in bilingual adolescents could be expressed, characterised and delimited, in order to enhance our understanding of how various prerequisites effect literacy development and to facilitate identification and handling of dyslexia in bilinguals. The study is based on data from ten Spanish-Swedish speaking adolescents with reading and writing difficulties. The multiple case study perspective has made it possible to investigate a number of linguistic and cognitive parameters in both languages. Furthermore, comparisons were made with a bilingual group of ten Spanish-Swedish speaking adocelscents without reading and writing difficulties as well as with a group of ten monolingual Spanish and Swedish speaking adolescents with dyslexia. Dyslexia has been defined and delimited in a bilingual dyslexia profiles continuum. This continuum is one of the significant theoretical-methodological contributions of the thesis. Another important contribution is the research design, that is, the use of a bilingual matched comparison group (without reading and writing difficulties) as the norm. Furthermore, quantitative and qualitative analyses have been summarised as various profiles, such as reading profiles, writing profiles and oral discourse profiles. The results are discussed on both group and individual levels and show that language dominance ha a major impact on the manifestations of the reading and writing difficulties. The differences between the two orthographies are also of importance. Furthermore, advantageous results in both languages co-vary with extensive L1 education in the bilingual participants both with and without dyslexia.