The goal of this chapter is to show that production planning and lot-sizing models are not contradictory with a Just-In-Time (JIT) approach. Characteristics of these models are discussed to explain how they fit with JIT concepts. We first underline the importance of reducing setup times/costs in both lot-sizing and JIT models. We show that the definition of setup time/cost reduction can be different depending on the problem. We then discuss when demands cannot be delivered at their due dates (backlogs or lost sales). The relevance of time windows in a JIT environment is presented, followed by bounded production and inventory. The importance of capacity constraints and their impact on JIT systems are also discussed. Finally, the last section of the chapter is devoted to the interest of integrating lot-sizing and scheduling in a JIT approach.