Assessment has become a major issue in several fields (economy, health, education…). In teaching languages, teachers often measure students' language competences. We will first explore the field of assessment and identify the main characteristics of an assessment situation in foreign language teaching: what is assessed (which competence or knowledge)? when? by who? For Whom? for what learning or social purposes? We will discuss the technical (and also ethical) choices you have to make when you are designing an assessment or correcting learner’s production and analyse the advantages and limits of different assessment tools. We will then focus our attention on this particular kind of assessment known as formative.