Abstract: In this paper we investigate basic properties of various life distribution classes: IFR,IFRA,NBU,DMRL,NBUE,HNBUE and their dual.1.closureness under various reliability operations;2.closureness under shook models;3.closureness under weak convergence;4.characterizations of L-S transforms of life distribution classes;5. characterizations of total time on test transforms of life distribution classes;6.proxi mity between the exponential distribution and an arbitrary distribution in a certain life class;7.reliability bounds in a certain life class.Many of results are obtained recently.