Many people believe that aliens exist and some also believe that aliens are visiting our planet regularly. There are certainly thousands of sightings and encounters reported each year, although how many of these can be substantiated is not clear. Certainly most if not all are not real alien sightings, and everything from sir force test planes to weather balloons are blamed. Whether aliens would have visited or not does however struggle under scientific scrutiny.
The notion of aliens existing has gone from laughable to an almost certainly over the last few years as scientists have furthered out understanding about the universe. There are billions of stars in our galaxy alone what have planets, and of those certainly there are many which will be capable of supporting life. On top of this there are billions of other galaxies as well, so the chances are that there is definitely life somewhere else in the universe. Not only this but it seems likely that there as once life on Mars billions of years ago.
If life on mars was established as a fact, even only bacterial, then it would certainly mean that there is life elsewhere also. Despite the fact that some planets are uninhabitable, if two out of the nine (or eight) planets in our solar system, then similar ratios might be expected fur supporting life elsewhere. Therefore the relevant question seems to be whether aliens have ever visited Earth rather then whether they actually exist in the first place.
The distances that aliens would have to travel in order to reach earth and to spy on us from the skies is billions of miles. That being the case they would likely have to travel at the speed of light, or would have had to have mastered stasis for long journeys. Both instances also dictate that aliens capable of visiting us would be thousands of years beyond our own technology, and would certainly be capable of hiding themselves when they got here.
It does seem odd that the people who tend to report seeing aliens and being abducted by them are invariably not otherwise successful people. You never hear about athletes or rich people being abducted, it always seems to be toothless backwater yokels and the mentally deficient. That isn't to say that there aren't any aliens as a result of the fact that people lie. However it would seem likely that aliens would reveal themselves to the governments or that they would abduct only the very best subjects to perform their tests on.
If aliens who were millions of years in advance of us were to want to conduct experiments on humans, then they would not need to abduct people regularly. They would be more than capable of either cloning or breeding humans for themselves, and would not likely return humans who would tell everyone about their experiences. In the same way that scientists have little regard for lab rats, advanced aliens would probably simply kill any humans they were finished with, and wouldn't put them back in their beds afterward.
Similarly many sightings appear to be rather obscure in nature, and people tend to see something unusual in the sky and assume it might be an alien. More likely they have simply seen a test flight for a military plane, which is why a lot of sightings of UFOs tend to happen near military installations. As far back as the 1930s the Nazis were developing a flying sauces, and had achieved flight with their prototype before the plug was pulled on the project. If saucers were a reality over 70 years ago, then certainly all manner of different designs are being made in secret today.
The other thing to consider is that Aliens would certainly be able to see whatever they wanted to see of Earth from deep in space. If scientists on Earth today can see distant planets, and spy satellites can see everything down to picking out individual people, then advanced aliens would be capable of more. Basically implying that they wouldn't need to come very close to be able to observe what is happening o the planets surface. They might even be advanced enough to be able to see down to the chemical makeup of single cells from outer space.
Aliens capable of observing things in this way would therefore have little need to regularly fly low over populated areas which is what they are reported as doing thousands of times each year. Certainly even if they did want to fly low, then they would be more than capable of hiding themselves from humans below. Whether they would use a cloaking device, cloud generator or other means remains speculation, but if they didn't want to be seen, then they simply wouldn't be.
Aliens visiting in antiquity is another popular idea that seems to come up every now and then, and there is some merit to this idea, although not concrete evidence. There are sites around the world which seem too advanced for the time, although of course knowing how things were built all those years ago isn't possible. Many people believe that the Easter island statues, Stonehenge and the pyramids are alien built. However more recently all of these building methods have been revealed by discoveries in the immediate areas, and all are certainly human.