There is a high level of uncertainty associated with the processing times of remanufacturing operations. The uncertainty in the quantity, quality, and timing of returned products further complicates the analysis of remanufacturing systems. Hence, researchers have developed various models to deal with the uncertainty problem. Based on these models, we can categorize uncertainty in remanufacturing into two main categories: internal uncertainty and external uncertainty. The variations within the remanufacturing process (viz., yield rate, station failures, remanufacturing lead time) are considered as internal uncertainty. External uncertainty arises from the factors outside the remanufacturing process such as the timing, quantity and quality (reusable rate) of the returned products, the timing and level of demand, and procurement lead times of new parts/products.