In this study some mathematical models in reliability systems were investigated, such as series , parallel , series parallel , parallel series and complex systems by using some methods such as ; path tracing method, reduction to series element method ,minimal cut method , event space method , composite method and decomposition method . A comparison was done between those methods to get the best reliability. Reliability allocation had been studied .The aim of it is to establish a goal or objective for the reliability of each component. Two methods had been used to find the reliability...