Feeble Interactions

The large hadron collider and other collider experiments are designed to discover hypothetical new particles with prompt decays. Almost stable and very long-lived particles are constrained by cosmological and astrophysical observables. In between these two extremes is a sensitivity gap for particles with moderate lifetimes. The focus of the talk is on different mechanisms which can lead to particles that are hard to detect and ideas to search for them and close the gap.