The issue of language of instruction (LOI) and its effects on education in Tanzanian secondary education has been widely researched since the early 1980s. In 2009, the Ministry of Education and Vocational Training proposed a new education and training policy that allows English to be used as LOI from nursery school to tertiary education. The proposed policy goes against what researchers in this area have recommended over the years. In the light of the proposed policy, the author of this article felt the need to review studies done on LOI in Tanzania from 1974 to date, aiming to eliminate or greatly reduce the negative effects of the policy on education in Tanzania. Quoting examples, the paper demonstrates students' levels of proficiency in English; suggests reasons why governmental policy has over time ignored research findings; and recommends as well as proposes the way forward.