The question of whether every finite group is a Galois over the rationals is still wide open. One fruitful method developed in the last 30 years has been rational rigidity. This is a property of the group and gives the group as a regular Galois group over Q(t) and so also over Q. We will discuss this method in two interesting cases for E8 and F4 over prime fields answering a conjecture of Zhiwei Yun. This is joint work with Malle (for E8) and Lubeck and Yu (for F4).