A core-collapse supernova event begins with a nearly spherical hydrodynamic implosion
of the core of a massive star and ends with a hydrodynamic explosion as an aspherical
shock wave expands through the stellar envelope.  The breaking of spherical symmetry
is both critical for driving the explosion as well as determining the characteristics
of the supernova.  The origin of asymmetry may arise in the first few hundred
milliseconds after bounce when the nascent shock wave is susceptible to the spherical
accretion shock instability, or SASI.  We describe the linear and nonlinear growth of
both an axisymmetric mode (l=1) and a non-axisymmetric mode (m=1), and argue that
this instability is a growing pressure wave driven by the dynamic response of the
accretion shock.  Using three-dimensional hydrodynamic simulations we show that the
non-axisymmetric mode of the SASI dominates the nonlinear evolution at late times.