So I bit the bullet and just bought a 15-inch rMBP which appears to be working great. I also have an 24-inch external Dell monitor (M2412) which I know is also working as I have tested it with other computers. However, when I plug a mini-displayport to displayport cable into my computer's Thunderbolt connector I am not able to get the computer to output any signal to my monitor. I looked through system preferences, did some Google searches and am not turing up anything. This is the cable I bought: http://www.amazon.com/gp/product/B00C7RJQPY/ref=oh_details_o00_s00_i03?ie=UTF8&psc=1 Any assistance would be greatly appreciated as this problem is quite distressing. ---------- Ok, I am a real dummy. I downloaded gfxCardStatus program, which I had never used before. I was searching frantically through the forum and came across something that indicated I needed to change the setting. I moved it to discrete and it started working. So now I have a follow-up question. Previously, I had it set to "Dynamic Switching". Isn't this supposed to mean that the software decides when to use the dGPU and iGPU based on the system needs? Essentially, why didn't it automatically switch over?