So far, this book has discussed how to design diffusers and absorbers, but mostly in isolation of where and how they are applied. This chapter starts by presenting some of the issues arising from the application of absorbers in rooms, especially the problems of translating absorption coefficients between the impedance tube, free field, reverberation chamber, and real room applications. It then proceeds to discuss how absorption and diffuse reflections are represented in geometric room acoustic models (GRAMs) and how this affects prediction accuracy.