A theoretical framework that distinguishes the knowledge, or competence, aspect of language proficiency from the skills aspect is outlined, and the factors in the language testing situation that affect performance on language tests are examined. The model is intended for use in assessing the construct validity of tests of language proficiency and in clarifying some terms and concepts that have been used to describe various aspects of language proficiency. The nature of the performance tasks and language competencies required by some widely used testing procedures are then examined within this framework, and the implications for psychometric theory and language testing research are discussed. (MSE)