The word "tachyon" have been much confused by public and abused by movie makers and television writers in Holliwood; as "faster-than-light particles", tachyons are perceived to be capable of creating such wonderful and terrible things as "wormhole" and "spacetime anomaly.".

In real world, tachyons are more commonplace than imagined, existence of which simply means that one is sitting at a local maxima or at a saddle point of the potential where the notion of particles as small excitations fails to serve as useful concept. In a field theory, be it describing superconductivity or the standard model, the folly of tachyon is easily remedied by a simple shift of variables. Two of the most famous particles, Cooper pairs and Higgs particle, are well known result of such an exercise.

In the current form of string theories, however, this simple change of variable is most difficult to implement in a reliable fashion, owing to its first quantized nature, and one must deal with tachyons more directly. Tachyons that had been thus avoided at all cost until recently, are now proving to be an effective tool in uncovering hidden structures behind string theories in great depth. In this talk, I will review physics of tachyon in the simplest term, illustrate appearance of tachyons in string theories, and outline newly emerging features of string theories found with help of such tachyons.