We study Quantum Chromodynamics (QCD) at hadron colliders. This is done in the context of general purpose Monte Carlo Event Generators. The aim of this thesis is to develop and implement a model for the so--called Underlying Event for the program Herwig++. We discuss multiple parton-parton scattering as source of the UE and show that our implementation is able to describe existing data from the Tevatron collider very well. Nevertheless we extend this model by a soft, non-perturbative component.