Problems related to the study of second language learners are described, and the factors affecting language development are discussed. Factors influential in second language learning include psychological, sociological, and affective aspects. Similarly, the measurement of language has been influenced by linguistic, psychological, and social theory. The rise of bilingual and English-as-a-Second-Language (ESL) programs has also resulted in new terminology for both language dominance and language proficiency with respect to limited- or non-English-speaking populations, and these terms have added to the misinterpretations and misconceptions already existing in the language field. The identification of language dominance patterns is primarily a diagnostic function, and does not necessarily indicate the language in which content should be taught. The formal language on which measures are frequently based may be unfamiliar to the student. In the press for classification, many errors are made in specifying the child's level of control of a second language. Language proficiency in one context may be inadequate in other contexts. Community and cultural group attitudes are important factors in second language learning. All these are viable considerations in language acquisition success and failure, but are often ignored in studying the language learner. (MSE)