We describe a study of the underlying event in jet and minimum
bias events using data from the CDF detector. The underlying event
contribution to the jet energy has been calculated in jet events and
compared to the results of two Monte Carlo programs: Herwig and Pythia. The
analysis has been carried out at two different center of mass energies: sqrt
s = 1800 and 630 GeV. For most observables, good agreement is obtained with
at least one of the Monte Carlo programs. Neither program describes all
features of the data.