To test intelligibility, a large number of tests have been developed. By means of such tests, the degree of intelligibility can be expressed in a single number, often the percentage of input that was correctly recognized by the subject. This chapter presents an overview of methods for measuring the intelligibility of closely related languages, and discusses their advantages and disadvantages. It focuses on spoken-language comprehension, but many tests can also be applied to the comprehension of written language. Methods for investigating mutual intelligibility can be taken from other disciplines, for example in the area of speech technology, second language acquisition, and speech pathology.