Gamification of learning--the application of game design elements to learning activities--is currently a hot, if controversial, trend in education. Proponents of gamification, on the one hand, claim that gamification leads to learning gains. They assert that gamification reinforces important skills in education, such as problem-solving, collaboration, and communication. Furthermore, they maintain that need for interaction in a gamified approach to education encourages students to play an active role in the learning process, thereby increasing student engagement in online forums, projects, and other learning activities. Detractors of gamification, meanwhile, argue that it derails learning with aimless distractions, adds unnecessary competition stress, and fails to take into account certain learners' pedagogical needs. Research on gamification is gathering momentum and promises to help adjudicate many of the issues raised in this controversy. We therefore conducted a second-order review to examine the evidence-based discourse on this aspect of gamification. We found that while the review literature adequately summarizes evidence in support of effectiveness in terms of cognitive, emotional/motivational, and behavioural outcomes, certain design issues remain unaddressed. When addressing effectiveness, a concern for how the nature of learning objectives and the quality of learning activities that are gamified is noticeably absent in the field. Furthermore, a contextual bias towards STEM courses limits the generalizability of evidence to other contexts. The reviewed literature further suggests a publication bias and an over-emphasis on positive effects. Nevertheless, recognising the general scope of the research, its theory, and evidence, will help instructors and curriculum designers interested in gamifying courses decide how to approach gamified course designs to use in a specific context. Highlighting current limitations in the evidence-based discourse may benefit the design of future research by drawing attention to the types of evidence that will help advance gamification in educational settings.