  It is one of the most remarkable features of quantum physics that
measurements on spatially separated systems cannot always be described by a
locally causal theory. In such a theory, the outcomes of local measurements are
determined in advance solely by some unknown (or hidden) variables and the
choice of local measurements. Correlations that are allowed within the
framework of a locally causal theory are termed classical. Typically, the fact
that quantum mechanics does not always result in classical correlations is
revealed by the violation of Bell inequalities, which are constraints that have
to be satisfied by any classical correlations. It has been known for a long
time that entanglement is necessary to demonstrate nonclassical correlations,
and hence a Bell inequality violation. However, since some entangled quantum
states are known to admit explicit locally causal models, the exact role of
entanglement in Bell inequality violation has remained obscure. This thesis
provides both a comprehensive review on these issues as well as a report on new
discoveries made to clarify the relationship between entanglement and Bell
inequality violation.
