For instance, continuing medical education is an educational form for resident doctors and interns who need to grow their practical skills in clinical diagnosis [7], and in-service teacher education is regularly implemented as part of the educator licensure requirement in many nations [8]. The typical forms of professional training include conference, lecture, workshop, and traineeship [9].  
Professional training differs from K-16 education in its educational focus and pedagogy. While K-16 education focuses more on students' learning outcomes and academic achievements, professional education is more career-oriented, concerning the cost-effectiveness of the training program. Consequently, K-16 education often relies on the student-centered pedagogy to promote higher-order thinking and meta-cognitive skills [10], whereas professional training is more aligned with skill development featured by direct instruction and trial-and-error practice [11]. The literature highlights the importance of professional training as it has a positive impact on employees' working attitudes, job performance, and knowledge acquisition [6; 12].  
The aforementioned characteristics of professional training highlight the potential of AR as a proper instructional technology for this particular context. First, AR enables the natural integration of virtual instructional content into the actual working environment [13], which can promote situational cognition and experiential learning. For example, [PERSON] et al. (2015) described an authentic AR-enhanced surgery environment where novice physicians can improve their neurosurgical skills through hands-on practice [14]. Second, AR can provide a variety of visual cues in the digital forms of symbols, text, animation, or 3D objects, which are known to facilitate procedural learning in professional training [15]. Third, AR can facilitate a shared learning experience in groups owing to increased visibility of virtual content [16]. The ability to accommodate group learning can further improve the accessibility and feasibility of AR-supported professional training. Lastly, the digital artifacts afforded by AR allow for easy creation, modification, and duplication, which can greatly reduce the training cost for trial-and-error practice.  
Recognizing the great potential of AR for developing professional competencies, this study systematically reviews the relevant literature published over the last two decades (2001-2020) with the purpose of extending our understanding of AR-supported instruction in professional training contexts. We synthesized the research findings through the lens of publication trend, application context, instructional design, and technical features, and determined the overall effectiveness of AR and its moderating factors through meta-analysis. Particularly, this study seeks to answer the following questions:  
1. What are the trends of publications and research types for AR-supported professional training?
2. What are the essential technological features and affordances of AR that support professional training, and how are they evolving over time?
3. What instructional strategies have been employed in the AR-supported professional training?
4. What is the overall effectiveness of AR application in professional training and what are the moderating factors?