This chapter deals with multi operation models that are different from the flow shop models discussed in the previous chapter. In a flow shop model all jobs follow the same route. When the routes are fixed, but not necessarily the same for each job, the model is called a job shop. If a job in a job shop has to visit certain machines more than once, the job is said to recirculate. Recirculation is a common phenomenon in the real world. For example, in semiconductor manufacturing jobs have to recirculate several times before they complete all their processing.