This volume presents fourteen new essays by leading figures in the fields of ancient philosophy and contemporary metaphysics, discussing Aristotle's theory of the unity of substances. This topic remains at the centre of metaphysical enquiry. The contributors examine the nature of essences, how they differ from other components of substance, and how they are related to these other components. The central questions discussed here are: What does Aristotle mean by 'potentiality' and 'actuality'? How do these concepts explicate matter and form, and how are they related to the actuality of substances? These questions are crucial to an understanding of the unity of composite substances and their identity over time. The aim of the volume is both exegetical and philosophical : to address central issues in Aristotle's Metaphysics, and to stimulate further investigation of the problems and controversies that arise from these. Contributors: Robert Bolton, William Charlton, Michael Ferejohn, Michael Frede, Kit Fine, Sally Haslanger, Aryeh Kosman, Frank A. Lewis, Julius Moravcsik, Charlotte Witt, Michael Woods, Theodore Scaltsas, David Charles, Mary Louise Gill.