This book describes the discourse of biology from a systemic functional linguistic perspective. It offers a detailed description of resources based on text analysis. The description reveals co-textual patterns of language features, their expressions through grammatical resources, as well as their functions in the disciplinary context. The book also applies the description to analyse student texts in undergraduate biology, revealing characteristics of language and knowledge development. Although the discussion in this book focuses on the discourse of biology, both the language description and the descriptive principle can be used to inform the examination of knowledge in academic discourse in general, making this key reading for students and researchers in systemic functional linguistics, discourse analysis, English for academic purposes, applied linguistics, and science education.