Benati provides clarity about the characteristics and notion of language proficiency in the field of second language acquisition. He looks at four areas of research paradigmatically related to the role of proficiency: theorizing and measuring second language proficiency; the dimensions of L2 proficiency; factors contributing to the attainment of L2 proficiency and attaining L2 proficiency in the classroom. It also contains a variety of research accounts about the specific factors which have an effect on proficiency together with a theorised measurement of proficiency in second language research. It will be required reading for researchers in applied linguistics and second language acquisition.