  In this talk I briefly explain the concept of the structure function of a
photon (the best known boson). Then I review some of the current experimental
evidence which confirms the existence of `strong' interactions of photon
suggested by this idea. I end by pointing out how the photon `structure' has
important implications for the interactions of high energy photons and hence
for the design of the next generation of the high energy $e^+e^-$ (linear)
colliders which are absolutely essential for locating the missing links in our
knowledge of fundamental particles and interactions among them.
