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Dissertation thesis elaborates and put into broader perspective of lean manufacturing two particular decision situations: optimization of the inventory levels between two adjacent production units and the competitive strategy for supplier in the supplier selection process. Discrete simulation is presented to be a suitable tool for determination of appropriate kanban card levels with respect to variability and overall system performance. Simulation optimization is introduced for kanban cards allocation in complex systems, such as multi-stage, multi-product production systems. Further, the lean procurement is presented as an integral part of lean manufacturing with significant influence on the just-in-time system. Stochastic model, which assess supplier competitiveness with the cost of further improvement, is developed.