How useful are preemptive schedules?

Machine scheduling admits two options to process jobs. In a preemptive mode processing may be interrupted and resumed later even on a different machine. In a nonpreemptive mode interruptions are not allowed. Usually, the possibility to preempt jobs leads to better performance values. However, also examples exist where preemptions do not improve the performance. This paper gives an overview of existing and new results on this topic for single and parallel machine scheduling problems.