This book chapter is based on a research conducted under the LOITASA (Language of Instruction in Tanzania and South Africa) project. An assumption prevails among policy makers in Tanzania that students entering secondary school are proficient in English, having learnt it as a subject for five years from Standard 3 to Standard 7. However, research finding do not support this assumption; instead findings indicate that students entering secondary education are not sufficiently competent in English to study across the curriculum through the English medium. Students however, are reasonably good at written communication in Kiswahili; unfortunately, the ‘English Only’ policy that is being used is predicated upon outmoded theories of second/foreign language acquisition, which discourage code-switching and fail to recognise the nexus between improved first language competence and the successful development of a foreign language. The quality of instruction in English is further hampered by the lack of physical facilities of curricular materials and of time.