The United States military services are modernizing their training and education curricula by leveraging advances in technology to deliver instruction that is more engaging and responsive to trainees' needs and better prepares them for the future fight. Adaptive training (AT), or training tailored to the strengths and weaknesses of individual trainees, is a promising technique to meet these modernization goals. The research literature, however, is sporadic and does not clearly prescribe best practices for its employment. Therefore, we conducted a meta-analysis to examine the effectiveness of various AT instructional interventions (i.e. adapting difficulty, feedback, scaffolding, etc.) on learning outcomes. There were 30 peer-reviewed publications included in the analysis. We grouped studies by the adaptive intervention examined and reported the associated effects on learning outcomes. Overall, the results revealed that the effectiveness of AT varied considerably across the instructional interventions. Specifically, studies that implemented adaptive difficulty techniques were the most effective, followed by adaptive scaffolding and remediation/test-out techniques. Based on these findings, we identify design recommendations for future AT systems.