  Magnetic fields have been observed in galaxies, clusters of galaxies and
probably in superclusters. While mechanisms exist to generate these in the late
universe, it is possible that magnetic fields have existed since very early
times. This thesis is concerned with methods to predict the form of such
imprints. We review in detail a standard, linearised cosmology before
introducing an electromagnetic field. We then consider the intrinsic statistics
of the magnetic stresses in two ways, analytically and via static realisations.
We construct the power spectra, some of which we present for the first time. At
the one- and three-point level we find significant intrinsic non-Gaussianities.
Finally we turn to the observable impacts a primordial magnetic field. Assuming
coherence, the statistics of the source can be mapped onto the CMB in a simple
manner. We demonstrate that our approach is valid by reproducing the signals
for Gaussian power law fields on the microwave sky. [ABRIDGED]
