I used to build computers in my youth. Overheating in CPUs becomes more frequent as thermal paste evaporates over time.
Thermal paste is cheap. You squeeze it out of a tube onto the CPU chip, then lock the heat dispersment metal back over the CPU. The fans then blow heat away.
Here is a video of how to do it in a 2011 macbook pro. http://m.youtube.com/watch?v=BqCsZKjwltY
In the meantime, increase the fan speed using the program suggested by previous posters in this thread.
If you let the CPU overheat to over around 65 degrees Celcius, the CPU will shut down torotect itself. After this the computer will often freeze, and even the power button might not respond (to force shut down, disconnect the battery).
Heres a tip: buy a set of tiny screwdrivers so you can open your laptop whenever you want. You will also need a good light an a desk that isnt covered in pizza boxes. Prepare your work area neatly. If you destroy any screw heads, my carpenter friend says you need to lightly hammer a flathead screwdriver into the top of the damaged screw to create a new indent. Dont reuse damaged screws - throw them away - much easier next time.
If you go to a genius bar, they might not think of this solution, so tell them that your CPU's thermal paste may have evaporated. This has cured many of my old IBM Compatibles since 1989.
Macbooks get extremely hot because they do not have a sufficient cooling system. Always angle your macbook a bit so it doesnt make full contact with the desk, and I strongly urge you to get a fan base in summer. Especially if you are taking your laptop o bed with you (I think most people use their computers in bed).
Summer often gets very hot for computers if your building doesn't have airconditioning. Refrigerators from kitchens heat the house, as do some light bulbs. I lived in a van and that gotreally, really hot in summer. The fan speed control program really helps a lot, but its always good to have cold air outside your computer too.
Im quite certain that the most permanent repair for heat creashing computers is the thermal paste I mentioned. Usually less than five bucks.