The constitution relation is discussed in some detail in this chapter. It is a three-place relation standing between an individual, a quantity of matter and a time, allowing for the typically changing constitution of individuals over time. The fact that times are intervals is accommodated and connections with the occupies relation, in terms of which coincidence is defined, are explored. Time-dependent analogues of the mereological relations are defined as triadic relations between two individuals and a time, but there are no corresponding analogues of the mereological operations. This has an import for the traditional problem first raised by Chrysippus’s story about Deon and Theon (Dion less a foot). Modal features of individuals and quantities are developed on the basis of fixed domains for times, regions and quantities and a variable domain for individuals. Times, regions and quantities all satisfy the Barcan and converse Barcan formulas and principles of mereological essentialism to the effect that what is and isn’t a part is so necessarily. But the corresponding principles don’t hold for individuals. An appendix criticising Van Inwagen’s understanding of parthood as time dependent concludes the chapter.