Online instruction has become increasingly utilized in K-12 education. Used by alternative school settings for multiple decades, the COVID-19 pandemic forced millions of students in public, charter, and private schools into online learning spaces in early 2020. Almost every student, including those most vulnerable to academic difficulties, engaged in core academic instruction utilizing online learning modalities for extended periods of time. The current study is a systematic literature review of the impact of online learning modalities on academic achievement in mathematics among K-12 students with disabilities, English language learners, and students from impoverished backgrounds. Retrieving research from two databases utilizing the Preferred Reporting Items for Systematic Reviews and Meta-Analyses, a total of 24 articles were reviewed. Overall findings, limitations of the extent research, and future research directions are discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]