Prepared for the National Hydrogen Association Annual Hydrogen Conference (NHA 2005), Washington, DC, March 29 - April 1, 2005Although reliability has not been evaluated for hydrogen energy systems, it is often assessed in other energy sectors. We investigated methods used to assess reliability in existing energy systems (specifically the electricity, natural gas, and petroleum sectors), and tailored these to form a suitable assessment methodology for hydrogen systems. Here we provide a brief background about some of these methods. Energy system reliability measures are broadly categorized according two general concepts: adequacy and security.