Typically there are citations filed along with the patent application, the purpose of which is to help add validity to the request for a grant or publication status. For USPTO filings; I thought that all patents were assumed invalid, and there had to be evidence provided to the contrary.
@Spinoniousv- It's 20 years from the Application Date, not the Grant date, the date the clock starts ticking is known as the Priority Date. Depending on the time it takes to get a Grant or 'Publication' status, many years may have already expired.
These terms are pretty much the same across many WIPO countries (World), including the EPO (European Patent Office).
Depending on industry, 20 years may not be all that much of protection considering the time and money invested in the research of the patent. Pharmaceuticals and Bio/health patents are good examples where a company may have invested 20+ years and billions of dollars into a patent.