In this paper I will review the debates on the choice of Language of Instruction (LoI) in formerly colonized countries, giving special attention to the United Republic of Tanzania. I will explore the relationship of LoI to local debates on quality learning, cultural identity, as well as the influence of global actors and development discourses on LoI choices. A particular attention is given to: (a) The strong evidence for superior learning when the medium of teaching and learning is a local language; (b) The implications of these LoI policies for quality education, cultural identity and rights in education. The paper is based on empirical research in Tanzania made in 2007 to 2012. My focus in this paper will be on the recent changes in LoI in Zanzibar. In 2010, Zanzibar began the implementation of a policy that will change important aspects of the curriculum in primary and secondary education, which among other changes will replace the current LoI, Kiswahili with English in the subjects of Mathematics and Science from Grade 5. Tanzania made an early choice to use Kiswahili as the LoI in primary grades, but that policy has been contested for many years, partly due to pressure from global agents such as the World Bank and other international institutions such as IMF, British Council and donors mainly from English speaking countries such as the United Kingdom and the United States. In the United Republic of Tanzania, I have interviewed government officers, academicians, policymakers, NGOs' staff and journalists to elicit their understanding of the aims of language and educational policies. I have also observed teaching and learning in classrooms with a focus on how the LoI affects the quality of both.