The book introduces the reader to an argument-based approach to validity as a way to improve test validation in Second Language Acquisition (SLA) research. Motivated by the need for practical suggestions for raising proficiency assessment standards in SLA research, it exemplifies the approach by validating two distinct score interpretations for a new Russian Elicited Imitation Test (EIT). Two empirical investigations with 164 Russian learners in the USA and Germany were conducted to evaluate the accuracy of the score interpretations associated with two distinct test uses. The EIT proved to constitute a reliable and valid instrument for differentiating between a wide range of oracy skills. The proposed cut scores enabled prediction of several levels of speaking and listening proficiency. The author concludes with implications for using the argument-based approach for validating assessments in SLA research, for the use of the developed Russian EIT, and for future research on Elicited Imitation Tests in general.