Tests today are usually based on a communicative view of language, with less focus on grammar. In this chapter, a study is presented that investigates whether there is a difference in terms of grammatical development between a group of test-takers who passed and a group who failed an oral language test. The study addresses theories of SLA and language testing, i.e., Processability Theory and the model of Communicative Language Ability, the construct of the test. Data from learners' test performances were analysed using PT in order to see whether there was a consistent relationship between the PT stage analysis and the results derived from the test. The comparison shows a clear difference between the test-takers who passed and those who failed in terms of grammatical development. This implies a correlation between grammatical development and communicative competence in general which may indicate that PT constitutes a useful basis for oral assessment.