  We study the physics of spontaneously broken family symmetries acting on the
third generation. Massless familons (or Majorons) $f$ associated with such
broken symmetries are motivated especially by cosmological scenarios with
decaying tau neutrinos. We first note that, in marked contrast with the case
for the first two generations, constraints on third generation familon
couplings are poor, and are, in fact, non-existent at present in the hadronic
sector. We derive new bounds from $B^0$--$\bar{B}^0$ mixing, $B^0 \to l^+
l'^-$, $b\to s\nu\bar{\nu}$, and astrophysics. The resulting constraints on
familon decay constants are still much weaker than those for the first and
second generation. We then discuss the promising prospects for significant
improvements from searches for $\tau\to l f$, $B\to (\pi, K) f$, and $b\to
(d,s) f$ with the current CLEO, ARGUS, and LEP data. Finally, we note that
future constraints from CLEO III and the $B$ factories will probe decay
constants beyond 10^8 GeV, well within regions of parameter space favored by
proposed scenarios in neutrino cosmology.
