ABSTRACT: This article reports on the results of research conducted by ACTFL on the construct validity of the ACTFL Proficiency Guidelines and oral interview procedure. A multitrait-multimethod validation study formed the basis of the research design and analysis, which included tests of speaking, writing, listening, and reading in French and English as a Second Language. Results from Rasch analyses are also reported. In general, the results provide strong support for the use of the Guidelines as a foundation for the development of proficiency tests and for the reliability and validity of the Oral Proficiency Interview. The paper includes a detailed description of the research methodology, instrumentation, data analyses, and results. A discussion of the results and suggestions for further research are also included.