Writing in science can be challenging for all learners, and it is especially so for students with cognitive or language-based learning difficulties. Yet, we know very little about how to support students with learning disabilities (LD) or who are English learners (EL) when asked to write for authentic purposes during science instruction. Therefore, we conducted a systematic review of 14 high-quality studies to identify effective writing instruction elements for students with LD, those who are EL, and for at-risk learners more generally. We analyzed the studies according to purpose, participants, dependent variables, and interventions. Then, we categorized instructional elements into two broad types of support: (a) cognitive skills and processes, and (b) linguistic skills and processes. Quantitative analyses showed students (regardless of disability or language status) who received structured cognitive instruction on text features demonstrated substantial growth in writing. Conversely, although language in science differs from everyday language, it is absent from this literature. Thus, our findings provide insights into necessary cognitive and linguistic supports for these students, and implications for designing effective writing instruction.