Abstract: "Dark matter with long-range interactions, mediated by a light or massless force carrier, appears in a variety of theories with different motivation. Examples include the self-interacting dark matter scenario, which is motivated by the observed galactic structure, as well as models constructed to explain observed astrophysical signals. Notably, even the Weak interactions of the Standard Model exhibit long-range behaviour if the interacting particles are heavier than a few TeV. An important implication of long-range interactions is the existence of bound states. The formation of dark-matter bound states in the early universe and in the dense environment of halos today affects the phenomenology of dark matter in many ways. I will briefly describe the implications of the formation of dark bound states. I will then focus on their effect on the abundance of thermal relic dark matter, and discuss the importance of bound-state effects in testing and constraining the WIMP dark matter scenario."