Quantities - such as weights, distances, durations, sizes, densities, intensities, charges, temperatures, amounts, frequencies, forces - play a central role in our scientific understanding of reality. On the one hand, natural sciences describe natural entities, as well as the laws of nature governing their behaviour, as essentially quantitative; on the other hand, social sciences make an extensive use of quantities in the mathematical and statistical investigation of social phenomena. Quantities populate our every day life as well, by constituting essential characteristics of the processes we are involved in or the entities we interact with, such as the temporal duration of our activities, the spatial distance between where those activities take place, or the price of our economic exchanges. Given this central role of quantities, it is surprising that little attention has been given to a direct investigation of the notion of quantity itself. Some efforts have been made recently to elucidate the nature of quantity, but these efforts appear to be mostly scattered and unsystematic and they fall well short of the standard reached in elucidation of other fundament aspects of reality such as those of qualitative, formal, mereological, essential, modal, and temporal attributes. Preliminary reflection on the problem already reveals an impressive number of outstanding questions. What kind of thing is a quantity? What ontological category do quantities belong to? If they are attributes, what kind of attributes are they - categorical, dispositional, comparative, absolute, etc.? And what sort of things possesses them? Objects, events, regions of spacetime? What kinds of quantities are there? What characteristic do all these different quantities have in common, and what distinguishes them within their ontological category? What can measurement teach us about quantities? What can contemporary physics in general, and quantum mechanics in particular, teach us about quantities? The overall aim of this project is to bring together a team of experts to answer these questions, and so to shed new substantive light on the nature of quantities. The project will involve three principal investigators - a metaphysician, a philosopher of science and a philosopher of mind - and two postdoctoral collaborators - one specializing in ontology and one in philosophy of physics - as well as a team composed of internationally renowned ontologists, philosophers of physics, historians of philosophy, logicians, epistemologists, philosophers of mathematics and experts of measurement science that will collaborate on different parts of the project. Special input is expected from philosophy science and philosophy of mind. Since the questions of the project concern the nature of quantities, properties, relations and processes as well as their classification, the main focus of the project is ontological in nature. And since the key notion of the project is central to science and technology in general, the outcomes of the project will be of interest to all scientific fields, by contributing to the theoretical foundations of their enquiries.