Stephen Mumford puts forward a new theory of dispositions, showing how central their role in metaphysics and philosophy of science is. Much of our understanding of the physical and psychological world is expressed in terms of dispositional properties - from the spin of a subatomic particle and the solubility of sugar to a person's belief that zebras have stripes. Mumford discusses what it means to say that something has a property of this kind, and how dispositions can possibly be real things in the world. They have seemed to many to reside on the fringes of actuality, waiting to manifest themselves; Mumford's clear, straightforward, realist account reveals them to be far less enigmatic, and shows that an understanding of dispositions is essential to an understanding of properties, causation, and scientific laws.