  The local Langlands conjectures imply that to every generic supercuspidal
irreducible representation of $G_2$ over a $p$-adic field, one can associate a
generic supercuspidal irreducible representation of either $PGSp_6$ or$PGL_3$.
We prove this conjectural dichotomy, demonstrating a precise correspondence
between certain representations of $G_2$ and other representations of $PGSp_6$
and $PGL_3$. This correspondence arises from theta correspondences in $E_6$ and
$E_7$, analysis of Shalika functionals, and spin L-functions. Our main result
reduces the conjectural Langlands parameterization of generic supercuspidal
irreducible representations of $G_2$ to a single conjecture about the
parameterization for $PGSp_6$.
