This chapter focuses on how multilingualism should be defined and how it can be assessed. One of the main points is that multilingualism should be viewed as a dynamic process rather than a state. The chapter elaborates multilingualism by considering why people are of become multilingual, what the role of language contact is, and to what extent multilingualism is a system that can grow or decline depending on the type and amount of use. A lot of research has been done on first and second language attrition and the data typically shows substantial retention rather than attrition. Measuring language contact is notoriously difficult, mainly because data may mispresent the real situation. Globally two approaches are used to measure language proficiency: standardized, mostly discrete point tests on the one hand and self-evaluations on the other hand. In dynamic assessment, the focus is on the learning potential of a learner rather than on their (in)abilities.