Abstract: Dark matter models predict the production of weakly interacting massive particles (WIMPs) in proton-proton collisions. Many of these theories are tested at the LHC using the ATLAS detector, in which events are characterised by large missing transverse momentum carried by a dark matter particle-antiparticle pair. These models predict that the dark matter pair may be produced via a new dark matter mediator particle or though an invisible decay of the Higgs boson. This search focuses on events where the hypothesized particles recoil against a Z boson decaying to e+e- or mu+mu-. In this talk an overview of the search will be presented, including the signal models studied, the estimation techniques used to measure Standard Model backgrounds, and the procedure used to set limits on the dark matter particles. Results will be presented using the 2015+2016 dataset of 36.1 fb-1 at 13 TeV centre-of-mass energy.