This research aims to explore the attempted implementation of mixed ability grouping in junior high schools in Taiwan and the challenges generated by individuals and groups to this policy. The mixed-ability grouping policy in Taiwan has been disputed for nearly thirty years, but the disputes have never been examined from a wider perspective that considers the evolution of the policy and the contexts the policy process resides in. This study thus attempts to understand the process of the mixed-ability grouping policy from a contextualised, politicised, long-term perspective within which not only the ideological and practical debates, but the contexts that shape the conflicts over time, are taken into consideration. The study is grounded in an analytical framework that allows for the exploration of the politically-driven mainstream educational ideologies, the power relationships between policy actors, and the cyclical policy process. The research methods adopted consider the timeframe, the contexts, the multiple policy actors and the interactions among policy actors and between contexts and policy actors within the policy. Documentary analysis is adopted to trace the policy process, the conflicts within, and the political, cultural, economic and societal contexts of the policy from its inception to today whilst a questionnaire survey and in-depth interviews are utilized to understand the attitudes and actions of educational authorities and school educators. Case studies are conducted in two junior high schools in order to learn about the dynamics, the conflicts, and the considerations of grouping practice within individual schools. The key findings of this thesis are as follows. First, the mixed-ability grouping policy in Taiwan has existed through two different political regimes, within which the different mainstream educational ideologies and power distribution among policy actors contribute to the distinctive policy process, interpretations of disputes and patterns of conflicts. Second, although the first-line educators recognise the advantages of mixed-ability grouping regarding discipline and resource distribution, their perceptions of pupils’ ability and teaching are in line with the assumptions of streaming, which contribute to educators’ conflicting attitudes and actions towards the mixed-ability grouping policy. Third, the senior high school entry system and the actions of parents and junior high schools together shape a hidden educational market within which the ‘disguised forms’ of streaming, such as the establishment of special classes, are valued by market players. The senior high school entry examination also profoundly influences educators who internalise the values embodied in the examination and perceive pupils’ ability and their own teaching mainly in terms of examination results.