The stagnation of National Assessment of Educational Progress (NAEP) Writing scores demonstrates the need for research-based instruction that improves writing for all students, especially English learners. In this article, we synthesize the literature on effective instructional practices for this diverse group of learners and describe how these strategies are leveraged in a teacher professional development program that has been previously shown to improve students’ argument writing. Then, we share results of a study that focuses on distinct subgroups of secondary English learners students to (a) determine their needs and challenges and (b) examine the impact of a cognitive strategies approach on rhetorical and linguistic aspects of writing at posttest. Results show English learners have considerable challenges with higher-order tasks involved in writing literary arguments and with the linguistic demands of academic writing before receiving the intervention. However, after receiving the intervention, using descriptive statistics and multiple hierarchical linear regression, we show that these students grew in the areas of presentation of ideas, organization, evidence use, and language use. For example, students designated as reclassified English learners (RFEP [Reclassified Fluent English Proficient]) and students who have even more limited English proficiency (designated as EL [English learner] here) show improvements in many aspects of writing, especially in their ability to write claims and use evidence. In contrast, improvements in language use components were more limited for both groups of learners. Moreover, some of the gains due to being in the treatment were significant enough to bring the average EL student close to parity or beyond their EO (English Only) / IFEP (Initial Fluent English Proficient) peers in the control condition at posttest. We conclude by discussing pedagogical implications for English learners.