Here are a couple of HDRI issues to be aware of: 1) As Anastasis20 commented, there are TWO places to check "Light throught transparency". The first one is in the render room at the top of the Property Tray. The second one is a little further down in the Global Illumination portion of the Property Tray. Both must be selected. 2) Resolution of the HDRI maps affect the quality of the render. Most of the maps available for free are low resolution. Strange lighting occurs with this. To help "disguise" the low resolution HDRI maps, you must increase the photon count in the Global Illumination portion of the Property Tray and increase the Interpolation Precision of the lighting. Interoplation basiclly allows Carrara to guess about the light instead of actually doing the math to figure it out saving time on the render. Increasing it forces Carrara to do more math and will increase render time. How much should you increase Photons and Interpolation? There is no specific answer. It's whatever looks best in your image based upon the HDRI map and design of your scene. Mark