  The conventional interpretation of quantum mechanics, though it permits a
correspondence to classical physics, leaves the exact mechanism of transition
unclear. Though this was only of philosophical importance throughout the
twentieth century, over the past decade new technological developments, such as
quantum computing, require a more thorough understanding of not just the result
of quantum emergence, but also its mechanism. Quantum decoherence theory is the
model that developed out of necessity to deal with the quantum-classical
transition explicitly, and without external observers. In this thesis, we
present a self-contained and rigorously argued full derivation of the master
equation for quantum Brownian motion, one of the key results in quantum
decoherence theory. We accomplish this from a foundational perspective, only
assuming a few basic axioms of quantum mechanics and deriving their
consequences. We then consider a physical example of the master equation and
show that quantum decoherence successfully represents the transition from a
quantum to classical system.
