Until 2013, Area 51 didn't officially exist. However in July 2013 the CIA publicly acknowledged it's existence and released previously classified documents. The facility is located in Nevada USA. A military airfield is located on the base near a large salt flat called Groom Lake at the facility centre.
Prior to 2013 the government didn't acknowledge the existence of the base and therefore it had no official purpose. Following the release of documents in 2013 the official purpose of the base prior to that time has always been for development and testing of secret aircraft and weapons. No further information has been released, and one can only assume that the base continues to serve the same purpose. A good detailed description of Area 51 and it's history can be found on Wikipedia.
Due to the secrecy surrounding the base, many conspiracy theories have evolved including testing of alien space craft, collusion with aliens, conflicts with belligerent alien species, underground tunnel systems, black projects, experimentation on captured aliens and more. Let me stress here that there is a multitude of information available on the internet regarding these conspiracy theories, and that the purpose of this page is not to delve into them too deeply, but simply to acknowledge that they are there. You can immerse yourself in their stories at your leisure, but be forewarned that there is very little consistency from one site to the next. I haven't come across a single site offering real evidence of their claims, but many of them are very interesting reading.
The Lockheed U-2 was one of the earliest aircraft tested at Area 51 by the US Air Force. The History Channel has a very good web page about the types of experimental aircraft tested at Area 51. Some well known aircraft developed and tested at Area 51 include the SR-71 Blackbird and the F-117 Nighthawk.
Another well done factual look at Area 51 can be found on the Stuff You Should Know Area 51 article.
The original runways at Area 51 were built in the 1940's by the Army Air Corps. In the early 1950's the Atomic Energy Commission owned the land for nuclear testing purposes. Later in the 1950's the CIA in conjunction with Lockheed acquired the area to develop high altitude surveillance aircraft, starting with the U-2 Spyplane. U-2 Spyplane testing along with more advanced plane designs to follow in subsequent years are thought to be responsible for the majority of UFO sightings around the area since the 1950's. In the late 1950's the government restricted the airspace over Area 51 and the Groom Lake installation.
The 1960's and 1970's see a lot of activity relating to defence contactors, development of spy technology and study of captured Soviet Migs. In the late 1970's testing began on the F-117 flat fuselage stealth fighter. In 1979 the CIA hands over control of the Area to the US Air Force.
The 1980's saw testing of the B-2 Stealth Bomber and an increase in speculation about Aliens with a claim in 1989 by Bob Lazar that he worked as a physicist back engineering Alien Spacecraft. The whole Bob Lazar episode is intriguing to say the least. There are so many claims and counter claims that it's easy to get tangled in the web of what can only be a fantastic mix of truth, lies, half truths and misinformation. Unfortunately Bob Lazar offers no physical proof other than his own testimony. The Angry UFOlogist will tell you why Bob Lazar's science can not work.
In the early 1990's conspiracy theories erupted with the advent of the world wide web. The Federal Government acquired an expanded area of land around Area 51 that had been the best viewing spots for public anomaly spotters, and the public had to find a new viewing platform over 40 km from the base.
So where does the evidence take us with regards to Area 51?
Sightings of strange objects in the sky abound. Does this mean there are alien spacecraft flying above Area 51 or does it simply mean that people are seeing things they haven't seen before and can't readily explain? Seeing something you can't explain doesn't automatically mean you are seeing aliens. We now know that since the 1950's the US Government has been testing advanced aircraft which looked very different to what people were accustomed to seeing in the sky, many of which were capable of flying at very high altitudes and very high speeds. Is it more likely that people seeing these planes being tested would think they are extraterrestrial, or that alien spacecraft were flying above Area 51 and assisting the US with technological advancement? I think the best case against the latter is that if aliens were indeed assisting with technological advancement, why did the technology still have to go through stages of development firstly with the U-2, then an F-117 and then a B-2? With alien technology wouldn't we have simply jumped straight to something beyond even the B-2? These incremental steps in advancement are probably the most compelling evidence for Area 51 being nothing more than a secret testing area for terrestrial technology.
Whilst we won't say categorically that there is no alien involvement at Area 51, there is no physical evidence for it. Like many other areas of controversy there are millions of ambiguous blurry photographs and videos and there are thousands of eyewitness testimonials. There are also countless attempts to fake proof. The reality however is that Area 51 is most likely a very exciting place with lots going on, but very terrestrial in nature.