Collective coupling of two-level quantum systems to a common electromagnetic mode gives rise to superradiant and subradiant states through interference between different emission pathways. In this talk, I will discuss collective dark states in cavity QED using the Tavis–Cummings model. I will show how photon emission and photon-number measurements can be used to distinguish collective spin sectors and entangled states. I will then discuss the effect of disorder in the spin–photon coupling and show that, although disorder changes the dark-state wavefunctions, the number of independent dark states remains remarkably robust. Finally, I will discuss how the dark-state number can serve as an order parameter for a collective many-body phase transition.