  In the first part of this work, I review the theoretical framework of
cosmological perturbation theory necessary to understand the generation and
evolution of cosmic microwave background (CMB) anisotropies. Using analytical
and numerical techniques, in the second part I describe the impact on the CMB
power spectra of the standard cosmological parameters (such as the
matter-energy budget of the Universe, its curvature, the amplitude and spectral
properties of the primordial fluctuations, etc.). I introduce the most general
type of initial conditions for the primordial perturbations, deriving a new
analytical approximation for the neutrino isocurvature modes.
  In the third part, I discuss the issue of extracting constraints on the
parameters of interest from the recent, high-quality CMB measurements,
presenting the relevant statistical tools and focusing on the Fisher matrix
analysis as a technique to produce reliable forecasts for the performance of
future observations. I then apply those tools to the study of several possible
extensions of the (currently) standard Lambda CDM model: the presence of extra
relativistic particles, possible time variations of the fine structure
constant, and the value of the primordial Helium mass fraction. I also use the
CMB as a tool to study the very early Universe, via its dependence on the type
of initial conditions: I relax the assumption of purely adiabatic initial
conditions and discuss the observational consequences and constraints on the
presence of general isocurvature modes.
