Error analysis is considered both a part of the methodology for the psycholinguistic investigation of the second language acquisition process as well as a part of the teaching-learning methods and materials re-evaluation process. The language acquisition process is a dynamic process governed by the principles of hypothesis formulation, testing and re-evaluation. Systematic errors reveal the state of a learner's approximative system at each successive stage of learning. In the investigation of the acquisition of "case" in German, an error analysis tentatively reveals five stages of development from a "caseless" system to a stable and coherent "two-case" interlanguage system. Both Corder's and Richards' categories of error types and Corder's algorithm are then used to illustrate the discovery of idiosyncratic sentences, the recognition and categorization of learners' errors, and the formulation of authoritative or plausible reconstructed sentences in data provided by learners of English as a second language. These analyses of errors should enable the teacher to supply the learner with the right kind of data for the formulation of more adequate concepts of the rules in the target language. (Author/CFM)