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. Issues discussed include: how diagnostic testing might most appropriately be developed; why the field has neglected to construct diagnostic tests; how proficiency has been diagnosed in the key areas of language: reading, listening, writing, grammar and vocabulary; the value of self-assessment as a key component in developing learners', and teachers', awareness of the complexity of language learning. The book ends with a consideration of and recommendations for future developments in the diagnosis of foreign language proficiency.