This dissertation focused on evaluating game-based learning in prevocational education. Though many researchers seem convinced of the potential of game-based learning, researchers also acknowledge that the empirical evidence for games as learning tools is ambiguous. Comparison between studies is difficult due to diversity in content, gameplay, and population (among other things). To distinguish between effective and ineffective practices and elements, and to optimize game-based learning, value-added comparison studies are required. Therefore, studies in the current dissertation adopted a value-added approach: a standard version of the game (i.e., an educational math game ‘Zeldenrust’) was compared to versions that were enhanced with different forms of instructional support (i.e., self-explanation prompts, collaboration, and faded worked examples). Three consecutive empirical studies investigated the effect of these forms of instructional support on the in-game performance and knowledge acquisition of prevocational students. It is important to highlight the outcome that learning was significantly enhanced in all three studies. The overall pretest to posttest Cohen’s d effect sizes for studies 1, 2, and 3 were 0.44, 0.46, and 0.46 respectively, which are regarded as medium-sized effects. These findings show that the intervention involving the educational game ‘Zeldenrust’ formed a solid foundation for learning about the domain of proportional reasoning. The results of the three studies show that further improvement of this environment was difficult, but possible. The investigated implementations of self-explanation prompts and collaboration did not result in improved effectiveness, but the addition of faded worked examples did. This seems to suggest that prevocational students benefit from continuous support that contains expert advice. Though game designers show a tendency to hide the instructional component and educational content in educational games, results of the current line of study suggest that a formal representation of the educational content in the game-environment could support students gameplay and learning.