International assessments show that 20% of adolescents cannot read simple texts with understanding. Despite this, research has focused on early reading in childhood and skilled reading in adulthood, neglecting reading development during adolescence. We report a longitudinal study assessing reading and vocabulary development at 12, 13 and 14 years in a sample of 210 adolescents who were unselected for ability. Word reading accuracy, word reading fluency, reading comprehension, receptive vocabulary and expressive vocabulary were assessed using standardized assessments. Latent variable models showed consistent rank order amongst individuals (high stability), significant progress over time, and evidence that achievement gaps between the least and most able adolescents were narrowing. Oral vocabulary knowledge and reading comprehension were best conceptualized as indices of a common language construct. Low levels of reading proficiency were also observed in a substantial proportion of this sample, underlining the importance of providing ongoing reading and language support during adolescence. Despite a widespread assumption that children can read when they leave primary education, international assessments show that 20% of adolescents are not able to read simple texts accurately and with understanding (Jerrim & Shure, 2016). These pupils will struggle to read independently, limiting access to the curriculum and hampering educational progress. Yet, very little is known about reading difficulties and reading development in adolescence, with research and theory focusing on reading in childhood and adulthood. Particularly lacking are longitudinal studies of adolescent reading: to our knowledge only one study has tracked reading within adolescence (see Francis, Shaywitz, Stuebing, Shaywitz, & Fletcher, 1996). We collected longitudinal data on word reading and reading comprehension at ages 12, 13 and 14 years. Spoken vocabulary knowledge was also assessed, as it is crucial for successful reading comprehension. Reading development in adolescence Beginning readers must learn how letters map onto sounds and then to read words and sentences accurately and efficiently. Once accurate and efficient reading is in place, a reader can allocate resources to reading comprehension, which is underpinned by spoken language comprehension (e.g., Gough & Tunmer, 1986). By late childhood, most children can read a range of texts accurately, efficiently and with comprehension, so it might be tempting to assume that little reading development takes place after this point. However, we know that the brain continues to develop during adolescence (Blakemore, 2018), and this includes the brain regions involved in reading (e.g., Ben-Shachar, Dougherty, Deutsch, & Wandell, 2011). Longitudinal studies of reading development in