What are physical quantities, and in particular, what makes them quantitative?This book articulates and defends an original answer to this important, insufficiently understood question through the novel position of substantival structuralism. This position argues that quantitativeness is an irreducible feature of attributes, and quantitative attributes are best understood as substantival structured spaces. The book first explores what it means for an attribute to be quantitative, and what metaphysical implications a commitment to quantitative attributes has. It then sets the stage to address the metaphysical and ontological consequences of the existence of quantitative attributes.