  Recent developments in the physics of extra dimensions have opened up new
avenues to test such theories. We review cosmological aspects of brane world
scenarios such as the Randall--Sundrum brane model and two--brane systems with
a bulk scalar field. We start with the simplest brane world scenario leading to
a consistent cosmology: a brane embedded in an Anti--de Sitter space--time. We
generalise this setting to the case with a bulk scalar field and then to
two--brane systems.
  We discuss different ways of obtaining a low--energy effective theory for
two--brane systems, such as the moduli space approximation and the low--energy
expansion. A comparison between the different methods is given. Cosmological
perturbations are briefly discussed as well as early universe scenarios such as
the cyclic model and the born--again brane world model. Finally we also present
some physical consequences of brane world scenarios on the cosmic microwave
background and the variation of constants.
