How is a patent invalidated?

When a patent is granted by the PTO there is what is known as a presumption of validity about the patent - that is, because the PTO granted the patent, the patent is assumed to be valid. Thus, it is later difficult to argue that a patent is actually invalid because this deference is given to the decision made by the PTO in granting the patent. So typically, when one tries to invalidate a patent, they will look for prior art which was not found by the PTO before it originally granted the patent. Because this prior art was not used in the decision to grant the patent, the same level of deference is not given to the PTO in this situation. So if it can be shown that this prior art would have prevented the patent from being granted in the first place (for example, in light of the new prior art, the patented invention may not be novel), then the patent can be invalidated, and the patent owner loses any rights to protect his invention.