Recent years have seen a growing interest in grammatical variation, a core explanandum of grammatical theory. The present volume explores questions that are fundamental to this line of research: First, the question of whether variation can always and completely be explained by intra- or extra-linguistic predictors, or whether there is a certain amount of unpredictable - or ‘free’ - grammatical variation. Second, the question of what implications the (in-)existence of free variation would hold for our theoretical models and the empirical study of grammar. The volume provides the first dedicated book-length treatment of this long-standing topic. Following an introductory chapter by the editors, it contains ten case studies on potentially free variation in morphology and syntax drawn from Germanic, Romance, Uralic and Mayan.