Facilitating diagnostic competences is an important objective of higher education for many professions. This meta-analysis of 35 empirical studies builds on a conceptual framework and investigates the role of problem-solving, scaffolding, and context to foster diagnostic competences in learners with lower and higher professional knowledge bases. A moderator analysis investigates which type of scaffolding is effective for different levels of learners' knowledge bases, as well as the role of the diagnostic context. Instructional support has a moderate positive effect (g = 0.39; CI [0.22; 0.56]; p = 0.001). Diagnostic competences are facilitated effectively through problem-solving independent of the learners' knowledge base. Scaffolding types providing high levels of guidance are more effective for less advanced learners, whereas scaffolding types relying on high levels of self-regulation are more effective for advanced learners. [This article was co-written with the DFG Research group COSIMA.]