In this chapter, I clarify what it is for a composite object to exist at a time and over time by analyzing the conditions under which things compose it at a time and over time. Some things, the xs, make up an object, O, at a time t1 if and only if the xs are maximally bonded then, which is to say that the xs are simply bonded, clinging together, and include everything simply bonded to any of the xs. The object O that is composed of the xs at one time may be made up of other things at a subsequent time. Over time, however, objects may change in composition only incrementally (unless we allow for the possibility of an object being disassembled then reassembled). Incremental changes are ones in which the mass of what is added (removed) is small as compared to the object’s mass before the addition (removal).