  In this thesis, we study aspects of D-brane realizations of the Standard
Model. Specifically, we study orientifold models with rotation and translation
elements that break supersymmetry, provide the general consistency conditions
and derive the massless spectrum for these type of orientifolds. These models
contain in general anomalous U(1) gauge fields. The Green-Schwarz mechanism
cancels the anomaly and provides a mass term for the anomalous gauge fields. We
calculate the bare mass for supersymmetric and non-supersymmetric vacua and we
show that higher dimensional anomalies can affect the masses of the anomalous
U(1)s. Phenomenological aspects are also discussed. We evaluate the
contribution of the extra U(1) fields to the anomalous moments and it is shown
that this imposes constraints on the magnitude of the string scale.
