Certification provides a structure to manage, improve, and communicate sustainability performance. However, this systematic literature review of 93 articles shows that there has been little change in the research questions, theories, methods, and insights gained over 20 years to understand the motivations for functioning, and benefits of sustainable tourism certification. To support researchers to increase the rigor, novelty, and significance of their research, we categorize past research into three key stages of the sustainability certification journey: exploration, implementation, and outcomes, based on the Antecedents-Decisions-Outcomes framework. Disaggregating each stage of the framework into its corresponding challenges, actions, and competencies results in a 9-cell matrix that is used to analyze aspects that are well researched and identify where research gaps exist. The matrix shows that we have little understanding of the competencies needed by businesses to explore, implement, or exploit certification programs, and we know little about the processes involved in making internal changes in a business to comply with certification. This review's matrix approach helps identify directions for further research and practice. It emphasizes the need for (1) theory-driven research questions and robust methodologies to understand the implementation decisions taken by applicants, and (2) measuring the actual impacts of sustainability certifications.