  The aim of this set of lectures is a systematic presentation of a 1+3
covariant approach to studying the geometry, dynamics, and observational
properties of relativistic cosmological models. In giving (i) the basic 1+3
covariant relations for a cosmological fluid, the present lectures cover some
of the same ground as a previous set of Carg\`{e}se lectures \cite{ell73}, but
they then go on to give (ii) the full set of corresponding tetrad equations,
(iii) a classification of cosmological models with exact symmetries, (iv) a
brief discussion of some of the most useful exact models and their
observational properties, and (v) an introduction to the gauge-invariant and
1+3 covariant perturbation theory of
almost-Friedmann-Lema\^{\i}tre-Robertson-Walker universes, with a fluid
description for the matter and a kinetic theory description of the radiation.
