Abstract: Because of the Black-Hole Information Paradox, and the singularity, many physicists believed that GR must be modified to incorporate quantum effects, and that such modifications may affect black-hole spacetimes. The additional near-horizon structure might modify the boundary conditions for incoming gravitational waves, and lead to gravitational wave echoes. These echoes will then become the "smoking gun" of modifications to relativity. Therefore parametrizing gravitational echoes and searching for them will be an important way to quantify how “black” the black holes really are. In this talk, I will talk about the theoretic formulation of gravitational wave echoes, and point out the current issues associated with this idea.