This systematic review investigated the literature between 1990-2021 to identify trends regarding science, technology, engineering, and math (STEM) scientists conducting K-12 outreach. The review identified 50 publications that reflected the scholarship on scientist-led K-12 outreach. This includes literature on the effective strategies regarding scientists' outreach efforts and how scientist-teacher partnerships could be improved for more meaningful and impactful K-12 outreach. In addition to best practice recommendations, this review revealed patterns in outreach participants and barriers to effective scientist-teacher partnerships. The results of this study suggest that there is a need for more rigorous and published scholarship on scientist-led K-12 outreach so that scientists and K-12 stakeholders can better understand the best practices and barriers related to outreach. Additionally, the review calls for better integration of the perspectives of educators into educational outreach activities from the onset of outreach. These strategies may lead to more valuable scientist-led K-12 outreach programs that more effectively broaden participation in STEM, a major goal of broader impact activities.