  This thesis describes the development of two independent computer programs,
Herwig++ and Effective. Both of these programs are used for phenomenological
predictions of high energy physics. Herwig++ is used to simulate events as
measured at particle colliders. This thesis presents a new set of variables for
parton shower evolution. These new variables retain the angular ordering
feature of the variables found in the original HERWIG software, while improving
the Lorentz invariance of the shower and improving the coverage of the phase
space. Also developed is a new model for hadronization. By changing the
distribution of probabilities of cluster decays into hadron pairs this model is
able to enforce desired results, such as isospin symmetry or meson-baryon
ratios, more intuitively. These new developments are compared against existing
LEP data. Effective is used to generate the mass spectrum of supersymmetric
models. This program is able to provide the 1-loop effective potential and
1-loop mass matrices for an arbitrary N=1 supersymmetric model. This software
also is able to solve the renormalization group equations at one-loop for the
parameters of the model and, in turn, provide the scale dependent values of
these parameters.
