This paper reviews state-of-the-art technology in the development of secondary stationary and mobile hydrogen storage systems, in particular the use of activated carbons. A new concept called Modification/Metal Assisted Cold Storage (MACS) is described. This includes the improvement in the efficiency of activated carbons in two ways: one is modification of the acidity of activated carbon surfaces and the other is modification by metal impregnation on the activated carbon surface. Evaluation of system's performance and cost of the four major alternatives for hydrogen storage: pressurized cylinder, metal hydride, liquefaction, and storage on activated carbons is presented. The MACS system is demonstrated to be an attractive method for advanced hydrogen energy storage.