 Also of interest are arguments to the effect that gravity itself may play a role in quantum state reduction (Christian, 2001; Penrose, 2001; also briefly discussed below). A fairly comprehensive overview of the current approaches to quantum gravity can be found in Oriti (2009). In this entry we have chosen to focus upon those approaches that are both the most actively pursued and that have received most attention from philosophers. Let us now turn to several methodological and philosophical issues that arise quantum gravity research.