What do C-Tests really measure? This book describes the history and the theoretical background of C-Tests and gives a survey of research into C-Test validity, which leads to the central question: How much context do test takers need to process when they take a C-Test? The author answers this question on the basis of extensive material and data from over 800 subjects. It turns out that the C-Test construct is fluid: Test takers have to take extended context into account, but not everybody to the same extent. The book discusses the implications of the fluid construct phenomenon for language testing practice and research.