This book addresses the need for tests that can diagnose the strengths and weaknesses in learners' developing foreign language proficiency. It presents the rationale for, and research surrounding, the development of DIALANG, a suite of internet-delivered diagnostic foreign language tests funded by the European Commission. The word 'diagnosis' is common in discussions in language education and applied linguistics, but very few truly diagnostic tests exist. However, the diagnosis of foreign language proficiency is central to helping learners make progress. This volume explores the nature of diagnostic testing, emphasizing the need for a better understanding of the nature of appropriate diagnosis.