The purpose of this enquiry is to investigate decentralisation in Nigeria and its implications for teacher leadership. In this study, teacher leadership is defined as the process through which classroom teachers in formal and informal leadership roles exercise leadership. Teachers do this by recognising and celebrating expertise, providing for the. needs of their' students, supporting colleagues, responding to the needs of their context and challenging the existing culture and structures. Extensive literature on the topic already exists, which shows that teacher leadership is an important focus of research. Most studies, however, are limited to Western contexts, with little attention to how school leadership is perceived and defined in Asian and African societies. Furthermore, as there are no published studies on teacher leadership in Nigeria, this PhD study makes an original and unique contribution to the literature. The investigation addresses five research questions: (1) Why and to what extent has educational policy in Nigeria become more decentralised and with what implications for teacher leadership? (2) What forms of teacher leadership are in evidence, and how are these shaped by context? (3) What are teachers' perceptions about why they engage in teacher leadership activities? (4) How is teacher leadership facilitated? (5) What are the barriers to teacher leadership and how might these be addressed? The framework of this study is therefore set within the context of research on leadership, professional learning and policy implementation . The empirical part of the study was conducted in four phases. The first phase involved the analysis of government policy on decentralisation. This provided insights into government expectations of teachers' involvement in school leadership. In the second phase, a pilot study was conducted in preparation for the main study. The third phase started with the distribution of a qualitative questionnaire; a total of 241 copies were distributed to all teachers in the three government senior secondary schools participating in the study in urban, semi-urban and semi-rural settings. Some of the teachers who responded to the questionnaire were selected to participate in semi-structured interviews and observations. The fourth phase involved a multisite case study in the same three government senior secondary schools in Abuja. The fourth phase involved a multi-site case study with the use of varied methods such as interviews, observations and document analysis. The participants were teachers from different management levels: principals, vice principals, senior mistresses and masters, heads of departments and classroom teachers. The findings reveal that in Nigerian schools, teacher leadership takes a particular form shaped by context, in some ways it is similar and in some ways different from previous findings on distributed leadership. The differences suggest a modification of Spillane et al.'s (2001) model on distributed leadership based on contextual factors. A model for understanding teacher leadership in three case study schools is developed, involving people and situations within and outside the school. These include school contextual factors such as structures, cultural practices, resources, and leadership of the SLMT and classroom teachers. The outside school factors include government policy context, the local community, and professional associations. This study contributes to knowledge by examining teachers' leadership in a context that has not been previously researched, providing a detailed description of teacher leadership practices in three different settings. The study concludes with a discussion of the implications of the research findings and possible areas for future enquiries.