4.1 Introduction

Texts of varying complexities are central to language learning and literacy development. A recent document in the United States, widely known as the Common Core State Standards (www.corestandards.org), that sets the national standards for K-12 English language arts and literacy in history/social studies, science, and mathematics, for example, recommends that "all students must be able to comprehend texts of steadily increasing complexity as they progress through school" and that by the time students graduate from high school, they must be able to "read and comprehend independently and proficiently the kinds of complex texts commonly found in college and careers" (NGA & CCSSO, 2010, p. 2).

Developing advanced literacy, arguably the main goal of English education at the postsecondary level, likewise entails deep engagement with complex texts. In China's higher education context, students are expected to develop the ability to read and comprehend appropriately challenging academic and nonacademic texts from original English sources. For example, English students are expected to be able to comprehend texts of increasing difficulty as they progress through college (高等学校外语专业教学指导委员会英语组，2000). Specifically, by the time students graduate from college, they must be able to read (at a speed of roughly 1600 words in five minutes), comprehend, analyze, and interpret book reviews and newspaper editorials in mainstream British and American newspapers, as well as reasonably challenging historical biographies and literary works published in English speaking countries. For non-English majors, students are similarly expected to be able to interact with texts of increasing complexity over the course of their college education as they progress from basic competence through improved competence to developed competence (高等学校大学外语教学指导委员会，2017). An understanding of what makes a text complex or challenging is therefore crucial to providing instructional scaffolds to students in their interactions with complex or challenging texts.(https://www.daowen.com)

This chapter draws on SFL to offer a linguistic critique of text complexity as it is commonly (mis) conceived by scholars and teachers in the field of language/literacy education. It also identifies the linguistic sources of complexity that create potential comprehension difficulties. Understanding text complexity and text difficulty is key to enacting popular instructional practices such as matching readers with "appropriate" texts or modifying existing texts to meet students' perceived reading levels. A focus on language in understanding text complexity and text difficulty, while partial in the contemporary multimodal textual environment (Kress, 2003; Unsworth, 2002), is important because language is often referred to as the "hidden curriculum" of schooling (Christie, 2012) and consistently shown to be the primary source of variability in predicting reading comprehension (Stone et al., 2013; Uccelli et al., 2015).