There is more than one type of patent search. The examples on this page list some of the more popular types of searches, and the various databases that can be most helpful for effective searching.
Other pages in this guide contain more details and search tips for the individual databases that are listed.
This type of search is broad in its scope, but not meant to be exhaustive. It will identify representative patents in an area of technology along with information about companies active in the field. Can include newspapers, magazines and journal articles, and web sites.
This is the type of search that comes to mind when you think of a patent search - an inventor has an invention which s/he is interested in patenting, and wants to see if anyone has already patented something similar or identical. For this type of search, the patent date is irrelevant, since if the invention has been ever disclosed (in a publication, a website or a patent) it is not patentable. Remember the rule: must be new, useful, and unobvious.
When performing this type of search you only need to look at current patents because the point is whether one patent infringes on another. As with many searches, it is the CLAIMS that are most important in guiding decisions on how to proceed.Often called a "Freedom-to-Operate" or "Right-to-Use" or "Clearance" search.
Similar to a State-of-the-Art search, but more comprehensive. You want detailed information about a particular industry including: primary companies and their competitors, key technologies - including patents by subclass, potential licensing opportunities, etc.
In addition to the databases listed above, the USTPO has some specialized databases that can be quite helpful.
A type of Prior Art Search conducted after a patent is granted to attempt to prove that said patent was issued in error. The main reason for invalidity is that one or more of the patent claims should be disallowed on the grounds that they were not novel when the patent was filed. This could be based on other patents or non-patent literature (NPL) such as journal articles or advertisements.
If you have a patent number, it is easy to find the patent. It is also relatively easy to find patents assigned to a particular company, or to an individual inventor. The USPTO databases are well-suited for this type of search, as their fielded searches yield much more accurate results than other search engines.