Society for the Metaphysics of Science Session. Pluralist approaches defend kinds of models/explanations beyond the causal and mechanistic ones endorsed by many philosophers of science, including models/explanations backed by constitutive/compositional relations between entities in nature. Exciting new philosophical projects consequently arise focused on understanding compositional models/explanations and connected phenomena such as the compositional "levels", "downward causation" and "constraining relationships", or "reduction" and "emergence" that working scientists routinely discuss alongside such models/explanations. There are presently two radically different approaches to the latter topics. Within philosophy of science, the most popular approach is a "Pessimistic" one that doubles-down on the claim that there are only causal or mechanistic models/explanations and seeks to reconstruct, or replace, these scientific phenomena within that framework. In contrast, a minority, but growing, "Optimistic" approach seeks to understand levels, downward causation/constraint, or reduction and emergence, in the sciences against the background of a plural range of models/explanations, including compositional ones. The papers of the symposium work within the minority approach to broaden discussions in philosophy of science by providing Optimistic accounts of compositional levels, notions of downward causation/constraint, and the nature of reduction in the sciences.