Modality is a vast phenomenon. In fact, it is arguably a plurality of phenomena. Within it, one type of modality warrants distinctive interest in philosophy and, in particular, in metaphysics. In view of this, this Element has a first part devoted to modality as a general phenomenon, where different types of modalities are distinguished, and where the question of unification is raised. Following this, the second part is focused on metaphysical modality: the type of modality that is of distinctive interest in metaphysics, and thus for the series of this Element. In this second part, the overarching question is about the source of metaphysical modality, and the discussion here informs back, and is informed by, the question of unification from the first part. This title is also available as open access on Cambridge Core.