Long-lived particles (LLPs) are predicted by a variety of new physics scenarios. As new physics involving promptly decaying particles has not yet been seen at LHC, interest is growing in exploiting the unique and clean signature of LLPs. Background-free LLP searches performed by the ATLAS experiment in Run I of the LHC resulted in tight constraints on new physics. I will discuss the physics motivation and the history of these searches, and focus on the first Run-II search, which used a missing-transverse-energy trigger. Technical aspects of the search and prospects for future measurements, including at other experiments, will be discussed.