Weakly Interacting Massive Particles (WIMPs) are candidates for Dark Matter. WIMP searches at lepton colliders are complementary to searches at hadron colliders and direct detection, since they directly probe the coupling to electrons which a priori is independent of the coupling to hadrons. Like at hadron colliders, WIMP pair production can be observed via an additional visible particle, in particular a photon from initial state radiation. With this technique WIMP masses to nearly half the centre-of-mass energy can be probed. Polarised beams are essential to reduce Standard Model backgrounds and to characterise the properties of the new particles in case a signal is discovered. Prospects for a mono-photon WIMP study at the International Linear Collider will be discussed in the context of EFT. In addition, detector requirements critical to this analysis are discussed.