The discovery of the non-zero neutrino mass presents one of the few avenues for physics beyond the Standard Model of particle physics. Neutrinos are several orders of magnitude lighter than all other particles raising fundamental questions about their underlying mass-generating mechanism. One possible explanation for the smallness of the neutrino mass is the existence of a hypothesized heavy neutral lepton. This seminar will present searches for these particles in the high-energy proton-proton collision data collected by the ATLAS detector.