Black holes emit high energy particles which induce a finite density potential for any scalar field $��$ coupling to the emitted quanta. Due to energetic considerations, $��$ evolves locally to minimize the effective masses of the outgoing states. In theories where $��$ resides at a metastable minimum, this effect can drive $��$ over its potential barrier and classically catalyze the decay of the vacuum. Because this is not a tunneling process, the decay rate is not exponentially suppressed and a single black hole in our past light cone may be sufficient to activate the decay. Moreover, decaying black holes radiate at ever higher temperatures, so they eventually probe the full spectrum of particles coupling to $��$. We present a detailed analysis of vacuum decay catalyzed by a single particle, as well as by a black hole. The former is possible provided large couplings or a weak potential barrier. In contrast, the latter occurs much more easily and places new stringent limits on theories with hierarchical spectra. Finally, we comment on how these constraints apply to the standard model and its extensions, e.g. metastable supersymmetry breaking.

24 pages, 4 figures; v2: minor revisions for publication