Research into language acquisition is reviewed and implications for practice are discussed. The research stemmed from the State of California's mandate to use specific oral language instruments for identifying students who are limited English speakers. The development of a test to identify and place English as a second language students in appropriate classes is discussed. This test was also designed to give the teacher useful diagnostic information. The design and statistical analysis of the grammar portion of this test is highlighted. Empirical evidence supporting a developmental rationale for the acquisition of grammatical proficiency in school settings is evaluated. It is shown that the language learner is an active participant in the learning process and uses three psychological strategies (simplification/reduction, generalization/expansion, and differentiation/distinction) to move along a continuum of language performance. In addition, the acquisition process is integrative; that is, the learner is not acquiring the negative in isolation of other inflections. A properly constructed test permits a teacher to look at the incorrect choices students make across a number of structures and thus to gain a good indication of their stage of acquisition and the types of structures they are acquiring. (RW)