When a large number of molecules are simultaneously coupled to the cavity, the effective coupling strength is scaled by √ N where N is the number of molecules (as will be discussed below). This collective coupling allows for significant Rabi-splitting despite the vanishingly small cavity coupling per molecule. Consequently, there has been a recent strong push by the community to better understand the collective coupling phenomenon from a rigorous theoretical perspective. In recent years, there have been a number of theoretical advancements that allow direct simulation of the quantum dynamics of a single cavity mode coupled to many molecules or many cavity modes coupled to many molecules . In the previous three sections of this review (see Sec. 3, Sec. 4, and Sec. 5), the discussion has been fo-cused on the properties, dynamics, and chemical transformations enabled by coupling a single molecule to a cavity mode. As we will see, the conclusions drawn in these previous sections that operate in the single molecule limit cannot be directly applied to the more experimentally relevant case of many molecules coupling to the quantized field inside an optical cavity. In this section, we will review recent theoretical works that investigate the modification of chemical and physical properties of matter in the collective coupling regime. In Sec. 6.1 we review computational works that study modifications to photophysical properties, such as absorption, photoluminescence, transport, decoherence, and population dynamics. In Sec. 6.2, we discuss theoretical works that show that charge transfer reactions can be modified in the collective coupling regime. Next, in Sec. 6.3, we review works that demonstrate the possibility of modifying chemical reactivity in the collective regime as well as works that provide conceptual insights on such processes. Finally, in Sec. 6.4, we discuss the unresolved mysteries of vibrational polariton chemistry in the collective regime and review some interesting works that have attempted to provide a resolution.