The Cray Time Sharing System, also known in the Cray user community as CTSS, was developed as an operating system for the Cray-1 or Cray X-MP line of supercomputers. CTSS was developed by the Los Alamos Scientific Laboratory (LASL now LANL) in conjunction with the Lawrence Livermore Laboratory (LLL now LLNL). CTSS was popular with Cray sites in the United States Department of Energy (DOE), but was used by several other Cray sites, such as the .