Two-echelon repair system is a repair system with multiple operating sites and multiple repair facilities and that the repair facility for a failed item is assigned depending on the types of failures. The availability analysis of two-echelon repair systems is discussed in this chapter. In multi/two-echelon inventory model, discussions of the optimal supply of spare items were traditional and well developed. Although the availability of an item in each operating site is an important measure for operators and system managers,availability analysis of multi/two-echelon repair system has been limited to very small systems. The approximation method to obtain the availability of an item in each operating site is discussed. This method is based on the iteration method and applicable to large scale two-echelon repair systems. The basic model and some modified models considering multiple items, common-cause failures, priority resupply, disposal of failed items and the lateral resupply are discussed.