Teacher leadership, as a significant component of school change has become a topical issue in the last decades (Wenner and Campbell, 2017). In recent decades, teacher leadership has evolved from small-scale cooperation and instructional contexts to becoming a cornerstone of school reforms in the late 1980s and early 1990s, and eventually focusing on supporting accountability systems in the early 2000s. (Little, 2003). Current studies have made important contributions in the literature by identifying factors that precede teacher leadership, e.g., school structure, school climate, teacher personal characteristics, and they have also recognized the outcomes of teacher leadership, such as improved collegial relationships (e.g., Wenner and Campbell, 2017; York-Barr and Duke, 2004), enhanced teaching practices and increased student achievement (e.g., Martin and Coleman 2011). Despite such important contributions, the literature still lacks a common definition of teacher leadership which would support future research on the topic and development of teacher preparation programs. In order to develop responsibilities for teacher leadership, it is important to be clear on the definition of teacher leadership. The critique towards lacking a clear definition of teacher leader is supported by the York-Barr and Duke (2004) review which stated that the literature on teacher leadership is “largely atheoretical” (p. 291) and Wenner and Cambpell (2017) review which concluded that only 35% of articles published in the period 2004-2013 clearly state the definition they use for the term “teacher leadership” in their study. This gap in the understanding of teacher leadership necessitates additional empirical evidence to promote cohesive future research in this field. Considering the limitations in the literature, this review paper aims to add to the existing body of literature on the teacher leadership topic by understanding the current conceptualization of teacher leadership. More specifically, through the review of relevant articles, the authors will identify the definitions used for describing teacher leadership and while using the deductive method the authors will rely on the theoretical framework of Snoek et al. (2019) to understand the conceptualization of teacher leadership in the recent literature. Finally, considering the ubiquitous presence of the concept teacher leadership in the current literature, through the inductive method the authors will try to distinguish the most important outcomes of teacher leadership. Against this background, the study aims to answer the following research questions by reviewing the studies that examined teacher leadership as a central topic published in the period from 2018 to 2023:

• How is teacher leadership defined and conceptualized in the literature?
• What are the outcomes of teacher leadership as identified in the literature?

To answer the research questions, this study utilized a systematic review method that synthesized qualitative information while extracting and summarizing themes related to teacher leadership definition and outcomes of teacher leadership that are manifested in the existing literature. This qualitative information is combined with quantitative data, specifically descriptive data, to provide a comprehensive overview of the existing evidence related to the conceptualization of teacher leadership against the Snoek et al. (2019) theoretical framework. To select studies for review, the authors adhered to the PRISMA approach developed by Moher et al. (2009) and applied specific inclusion and exclusion criteria. Three main themes guided the criteria, namely main focus, subject focus, and publication criteria. Firstly, studies had to primarily focus on teacher leadership to qualify for review; those only briefly mentioning teacher leadership were excluded. Secondly, eligible studies concentrated on teachers with teaching as their primary role, excluding those exploring teacher leadership from the perspective of principals or school administrators. The focus was also on K-12 teachers due to their strong classroom responsibilities. Thirdly, the studies considered for review needed to be either empirical or theoretical and peer reviewed. Policy analyses and non-peer-reviewed publications were excluded. Furthermore, eligible studies were required to have been published between 2018 and mid-2023 and be in English.

The results of this review demonstrate that there is still a lack of a common and consistent definition of teacher leadership in the literature. A clear understanding of the concept would help in better preparing prospective teachers for the new roles that the teaching profession demands for overcoming the daunting challenges that schools face nowadays. Most studies included in the review relied their work on the already existing definitions of the concept in literature. The definitions of York-Barr and Duke (2004) and Katzenmeyer and Moller (2001) were identified by the authors as the most used definitions in the studies published in the period 2018 –2023. Other studies either utilized definitions of other authors, developed their own working definitions based on the existing literature, or came up with definitions after analyzing first-hand data via qualitative or quantitative methodologies. Regardless, the studies reviewed revealed that teacher leadership is mostly conceptualized more as an informal and individual form of leadership. Whereas, the outcomes and impact teacher leadership causes can be at the school level e.g., school development, innovation and change, and transformation of school culture, at the teacher level e.g., professional development, collegial collaboration, improved instructional practice and self-efficacy, and at the student level e.g., student achievement and student motivation. This study points to the need to advance the debate on the occurrence and projection of teacher leadership within schools by combining the various variables of forms it occurs and the purposes it is meant to serve.

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