  We study interactions between the dark solitons of the parametrically driven
nonlinear Schr\"odinger equation, Eq.(\ref{NLS}). When the driving strength,
$h$, is below $\sqrt{\gamma^2 +1/9}$, two well-separated N\'eel walls may repel
or attract. They repel if their initial separation $2z(0)$ is larger than the
distance $2z_u$ between the constituents in the unstable stationary complex of
two walls. They attract and annihilate if $2z(0)$ is smaller than $2z_u$. Two
N\'eel walls with $h$ lying between $\sqrt{\gamma^2 + 1/9}$ and a threshold
driving strength $h_{sn}$ attract for $2z(0)<2z_u$ and evolve into a stable
stationary bound state for $2z(0)>2z_u$. Finally, the N\'eel walls with $h$
greater than $h_{sn}$ attract and annihilate -- irrespective of their initial
separation. Two Bloch walls of opposite chiralities attract, while Bloch walls
of like chiralities repel -- except near the critical driving strength, where
the difference between the like-handed and oppositely-handed walls becomes
negligible. In this limit, similarly-handed walls at large separations repel
while those placed at shorter distances may start moving in the same direction
or transmute into an oppositely-handed pair and attract. The collision of two
Bloch walls or two nondissipative N\'eel walls typically produces a quiescent
or moving breather.

Pattern Formation and Solitons