Project B3a will explore the phenomenology of dark sector models that address the dark matter problem by introducing new particles with very weak couplings to the Standard Model but potentially strong interactions between them. We will calculate predictions for collider signatures, such as dark showers or displaced vertices, and for observables at flavour experiments, such as rare decays. These predictions will then be compared to experimental data and combined with cosmological and astrophysical observations in order to perform global fits of dark sector models.