Demand is very sensitive with respect to different aspects of market. Keeping this in mind, we have discussed optimal inventory policies for different demand structures like constant demand, stock-dependent demand,price-sensitive demand, trended demand and quadratic demand. Players are always interested in making more profits and for this purpose they offer trade credits. So we have incorporated different strategies like trade credit linked to order quantity and two-level trade credits. Most of the products are deteriorating in nature. Considering this fact, in inventory models we have incorporated constant deterioration rates of items. In some models, we have incorporated the maximum lifetime of the items as well. Due to disruption, production is not always so smooth to produce goods regularly as per the demand. So, we have studied manufacturing processes with disruption. To provide quality product to the end customers is the prime objective of the manufacturer for reputation inthe market. Keeping this in mind, we analyzed a model for returned/reworked inventories during Imperfect production processes.