  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.
