Language testing is the practice of evaluating an individual's proficiency in using a particular language. There are two main types of assessment: formative assessment which checks student progress, and summative assessment which measures achievement at the end of a term. There are five common types of language tests: proficiency tests which measure overall ability, achievement tests related to course content, diagnostic tests which identify strengths and weaknesses, placement tests for assigning students to class levels, and direct/indirect tests. The effect of testing on teaching is known as backwash, which can be harmful if not aligned with course objectives, or beneficial if tests influence instructional changes.