Reflective/refractive surfaces. My understanding is - and I could be WAY wrong - that Maximum Ray Depth controls how many transparent materials that a ray is allowed to pass through before the program determines the final appearance/color of whats on the far side. So, when you have complex transparency situations, like you might get with an eye or with some of the layered Poser hairs, this can cut down how much time it takes to think about that stuff...now it may reduce the fidelity of the output, but I haven't really noticed anything. All that said, that's what I think is going on. I've seen a slight reduction in render times when I turn that down. I could be wrong as to exactly why. TomD