Sure, by definition, an unidentified flying object is any flying object that has not (yet) been identified. For the purposes of this discussion, I exclude from the meaning of "UFO" all natural phenomena such as meteors, lightning, etc and all human-caused phenomena such as aircraft, spacecraft that originated on Earth, balloons, flares, etc. Instead, I mean exclusively extraterrestrial spacecraft.

First of all, the complete lack of physical objects from UFOs is convincing. Surely, if all of these flights are going on constantly, some nuts or bolts or whatever must have fallen from a UFO and been found by now. This goes double for any crashed UFO, in which case there would be thousands of pieces distributed over square kilometers of area that could never be collected completely, so some would always be available to the diligent searcher.

Secondly, there are automatic meteor cameras scattered about the planet that record the arrival of any meteor bigger than a pea that comes into their field of view. There are dozens of professionally operated arrays of these cameras, and thousands more operated by amateur astronomers. These NEVER record UFOs. When spectacular sightings are claimed, these cameras show either ordinary natural phenomena, aircraft, or nothing when they are scanning the very same area of the sky as the claimants. There MUST be a reason.
See
http://www.arm.ac.uk/meteor-cam/detections/
http://aquarid.physics.uwo.ca/all_sky.htm

Thirdly, the absence of sonic booms in cases of low-flying UFOs moving at "fantastic speeds". Even an object as small as a 5.5 mm rimfire bullet leaves a clearly audible sonic boom behind it. Claims are made that some "light" took off and moved at a speed far higher than the familiar speeds of commercial aircraft. Since commercial aircraft cruise at about Mach 0.85, anything much faster would need to be supersonic. Here I am not discussing the unknown characteristics of the alleged spacecraft, but the very well-know characteristics of air. We know all about it. We know, in particular, than any solid object traveling though air at Mach 1.0 or more leaves a sonic boom behind. In addition, daylight sightings NEVER include mention of a visible shock wave trace around the spacecraft. These are easily visible on supersonic aircraft in humid conditions at low altitudes.
Here is what a real aircraft looks like at supersonic speeds in humid atmospheres:

In the fourth place, the related absence of condensation trails. Under appropriate atmospheric conditions, aircraft produce condensation trails even when traveling well below the speed of sound. This is so commonplace that I ask you only to look up sometime at jets flying far overhead. UFOs never produce condensation trails.

My conclusion: UFOs (in the sense specified above) simply do not exist.

David F Mayer

Good general reference at http://www.debunker.com/ufo.html