Tanzania, like many other multilingual communities globally, has not eluded the problem of Language of Instruction (LOI). As a result, nearly five decades, English, the LOI in secondary schools and high education, has been a "scapegoat" of many secondary school students' academic failure. The book shows clearly that English Language Proficiency (ELP) predicts future academic success of students. However, the influence is not as great as the proponents of Kiswahili exaggerate it to be; this is to say, it is the non-ELP factors that affect performance greatly. The author therefore, concludes that the use of O- level and A-level secondary school results alone, as it is done in Tanzania and elsewhere, to predict one's capacity for secondary and higher education respectively is not enough since academic performance is an interplay of a number of factors. The book is invaluable for language teachers, teachers in training, University students,language policy makers and curriculum developers.