Errors in second language learning are viewed as evidence of the learner's hypotheses and strategies about the new data. Error observation and analysis are important to the formulation of theories about language learning and the preparation of teaching materials. Learning a second language proceeds by a series of approximative reorganizations which are the learner's systematic and autonomous version of the target language. The notion of approximative systems may be extended to various types of so-called reduced or simplified types of speech, such as immigrant language and pidgins. Four aspects of the simplification of surface structure evidenced by pidgins are discussed. Psychological process (latent psychological structures) that guide most second language learners differ from the latent language structure. Second language learning may involve reference to universals common to all types of language learning; features of phonology and grammar may be inborn. Four types of studies are described for the collection and analysis of data on learner errors: cross-sectional, longitudinal, narrowly focused tests, and direct elicitation. Four criteria which are generally used in the classification and analysis of errors and approaches for reducing errors are discussed. Some implications of a learner systems approach to language teaching are considered, and suggestions are offered for foreign language curricular design, teaching objectives, instructional materials, and teaching procedures. (SW)