Two-phase, gas-solid processes are extremely difficult to operate. Fluidized-bed reactor systems and fluidized-solids recycle systems are two of the more complex two-phase systems and, therefore, offer special challenges in getting them to operate well and keeping them operating. A study has shown that plants processing solids operate at only about 68% of design capacity in the first year of operation, whereas the industry standard for plants that do not process solids is about 90 to 95%. During the past two decades, various tools and techniques have been developed to assist in minimizing startup and operating problems in plants that react and/or transport solids. These are very helpful tools, and if applied correctly can be used with much success to solve problems. When plant downtime can mean a multi-million dollar loss, solving (and preventing) problems in order to keep the units operating becomes extremely important. Two of the tools that can be used to diagnose and troubleshoot problems in commercial, hot units are cold models and pilot plants. Cold models (models operated at ambient temperature) can often be used to simulate and solve problems occurring in the hot unit. They are popular because they can be constructed of clear plastic to allow visual observation of the flow system. They are also relatively inexpensive and can be constructed in a relatively short time. Pilot plants that were used for scaling up a process can also be used for solving problems. This is more the case if the problems are of a chemical nature or if operation at exact process conditions is required. Using pilot plants judiciously can lead to significant improvements in the operation of the process.