After trying all sorts of things (moth balls, tea tree oil, lights on 24X7, live trap, etc.) that did not work to evict squirrels (they are very clever) from my attic, I did a lot of research and found that this was the most recommended product. It is fairly expensive, and I could not find a used one, so I bought a new one. One thing that is definite is that squirrels hate it, and it works right away in a simple attic. In other words, if your attic is totally flat and the light is be able to reach everywhere, you are all set. If you have a complex construction (as I did), you have to make sure that you put it so that the light reaches the hole from which squirrels go in and out. That way once they are out they won't come back in. At that point you should close the hole.