This chapter deals with continuous review policies. This means that the inventory levels are followed continuously and that replenishments may be ordered at any time. If the demand is Poisson (or compound Poisson) it is easy to realize that it is sufficient to consider those moments of time when demands occur. In case of low demand, there are relatively few such events, and it is usually suitable to apply continuous review policies. On the other hand, in case of very high demand, continuous review can lead to high information costs and it may be more practical to review the inventory system periodically. This means that continuous review policies are of special interest for inventory items that naturally have a low demand like—for example, spare parts. The chapter also discusses available models dealing with continuous review policies for multi-level inventory systems and emphasizes the main characteristics of different approaches. Furthermore, two-level systems and consumable items are also discussed in the chapter.