In this paper, we consider the problem of scheduling hard real-time tasks subjected to preemption delays on a uniprocessor system. While most of the existing work focus on either reducing these additional delays or improving the system predictability by bounding them, we focus in this work on the problem of taking scheduling decisions while considering preemption delays. We first study the behavior of existing online scheduling policies such as rm and edf when accounting for preemption delays. Then we prove that there exists no optimal online algorithm for the problem of scheduling sporadic tasks with preemption delays. Eventually, we propose an optimal offline solution to the problem of scheduling tasks subjected to preemption delays using mathematical programming.