What are physical quantities and in particular what makes them quantitative? <p/>This book articulates and defends an original answer to this important insufficiently understood question through the novel position of <em>substantival structuralism</em>. This position argues that quantitativeness is an irreducible feature of attributes and quantitative attributes are best understood as<br>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<br>attributes.<br>