  We present a calculation of jet activity associated with pair production of
top quarks at hadron colliders and supercolliders. Our approach is based upon a
merger of $2\rightarrow 3$ matrix elements with initial and final state parton
showers. We tabulate expected multi-jet rates in $t\bar t$ events, and compare
with ${\cal O}(\alpha_s^3 )$ matrix element results and parton showers based
upon $2\rightarrow 2$ subprocesses. The merged calculation includes the correct
matrix element for the first leg of the parton shower, but also includes the
effects of approximate all-orders quark and gluon emission. We also evaluate
expected jet activity in top pair events at hadron supercolliders; in
particular, a comparison of the jet activity in the far forward region is made.
It is found that the merged calculation yields more jet activity in the forward
region than pure matrix element estimates, but also more jet activity in the
central region. This can be important for background calculations to
$H\rightarrow WW$ and strong $WW$ scattering processes.

High Energy Physics - Phenomenology