This study synthesizes existing research on the implementation of tutoring programs which we define as one-to-one or small-group instruction in which a human tutor supports students grades K-12 in an academic subject area. Tutoring has emerged as an especially promising strategy for supporting students' academic success with strong causal evidence finding large, positive effects on students' math and reading test scores across grade levels. Prior studies have reviewed this causal evidence of effects, but none have summarized the evidence on implementation. We iteratively developed search and selection criteria to identify studies addressing key research questions and synthesized these 40 studies which employ a range of research methodologies to describe how tutoring is implemented and experienced. We find that existing research provides rich descriptions of tutoring implementation within specific programs of focus, with most studies describing after-school tutoring and small-scale programs run by university professors. While few elements of implementation are studied in depth across multiple studies, common patterns emerge. Tutoring program launch is often facilitated by strategic relationships between schools and external tutoring providers and strengthened by transparent assessments of program quality and effectiveness. Successful tutoring implementation often hinges on the support of key school leaders with the power to direct the use of school funding, space, and time. Tutoring setting and schedule, tutor recruitment and training, and curriculum identification influence whether students are able to access tutoring services and the quality of the instruction provided. Ultimately, the evidence points to strong tutoring being driven by positive student-tutor relationships through which tutors provide instruction strategically targeted for students' strengths and needs driving towards a long-term academic goal.