  The phenomenon of resonant activation of a Brownian particle over a
fluctuating barrier is revisited. We discuss the important distinctions between
barriers that can fluctuate among "up" and "down" configurations, and barriers
that are always "up" but that can fluctuate among different heights. A
resonance as a function of the barrier fluctuation rate is found in both cases,
but the nature and physical description of these resonances is quite distinct.
The nature of the resonances, the physical basis for the resonant behavior, and
the importance of boundary conditions are discussed in some detail. We obtain
analytic expressions for the escape time over the barrier that explicitly
capture the minima as a function of the barrier fluctuation rate, and show that
our analytic results are in excellent agreement with numerical results.
