  In this thesis, we investigate quantum vacuum effects in the presence of
gravitational fields. After discussing the general theory of vacuum effects in
strong fields we apply it to the relevant issue of the interaction of the
quantum vacuum with black hole geometries. In particular we consider the
long-standing problem of the interpretation of gravitational entropy. After
these investigations, we discuss the possible experimental tests of particle
creation from the quantum vacuum. This leads us to study acoustic geometries
and their way of ``simulating'' gravitational structures, such as horizons and
black holes. We study the stability of these structures and the problems
related to setting up experimental detection of ``phonon Hawking flux'' from
acoustic horizons. This line of research then leads us to propose a new model
for explaining the emission of light in the phenomenon of Sonoluminescence,
based on the dynamical Casimir effect. This is possibly amenable to
experimental investigation. Finally we consider high energy phenomena in the
early universe. In particular we discuss inflation and possible alternative
frameworks for solving the cosmological puzzles.
