Gamification is now a well-established technique in Human-Computer Interaction. However, research on gamification still faces a variety of empirical and theoretical challenges. Firstly, studies of gamified systems typically focus narrowly on understanding individuals. short-term interactions with the system, ignoring more difficult to measure outcomes. Secondly, academic research on gamification has been slow to improve the techniques through which gamified applications are designed. Third, current gamification research lacks a critical lens capable of exploring unintended consequences of designs. The 14 articles published in this special issue face these challenges with great methodological rigor. We summarize them by identifying three main themes: the determination to improve the quality and usefulness of theory in the field of gamification, the improvements in design practice, and the adoption of a critical gaze to uncover side-effects of gamification designs. We conclude by providing an overview of the questions that we feel must be addressed by future work in gamification. Gamification studies would benefit from a wider use of theories to account for the complexity of human behavior, a more thorough exploration of the many opportunities coming from the world of games, and an ethical reflection on the use of game design elements in serious domains.