Comment by joshuaheard

If you consider the Drake Equation (http://en.wikipedia.org/wiki/Drake_equation), there are probably millions of alien civilizations in our galaxy alone that could send craft to our planet. If that is true, they would certainly observe and study us, much like we observe and study lesser animals. And to avoid harm to their subjects, the aliens would likely avoid detection. So, even though I think 90% of UFO evidence is terrestrial phenomena, the odds are in favor of alien race(s) visiting us, so it's possible some UFO evidence is of alien craft.