well. The theme concerns a certain tool that one can use to uncover certain non-perturbative effects in brane physics by concentrating on the case of six-branes. Thus, the above title was born out of the idea that I would review past work involving six-branes, present some puzzles presently under study involving six-branes, and describe the promising future use of six-branes in addressing what may at first seem like a completely unrelated question.

The feature that makes six-branes unique in string theory is that they admit smooth, horizon-free strong-coupling descriptions in terms of eleven-dimensional supergravity. In the case of the D6-brane, the lift to M-theory is the Kaluza-Klein monopole, while for the orientifold 6-plane [@Sei1; @Sei2; @Sen] it is the Atiyah-Hitchin manifold [@AH]. These results turn out to provide a handle with which to grasp a variety of non-perturbative effects in brane physics, and we display a selection of such results below.

It turns out that several of the results of interest involve the Hanany-Witten effect [@HW]. For this reason, we begin with a review of the smooth picture of this effect and then proceed to discuss what one can do with six-branes. Our first application is the construction [@GM] of 