  Particle physics models where there are large hidden extra dimensions are
currently on the focus of an intense activity. The main reason is that these
large extra dimensions may come with a TeV scale for quantum gravity (or string
theory) which leads to a plethora of new observable phenomena in colliders as
well in other areas of particle physics. Those new dimensions could be as large
as millimeters implying deviations of the Newton's law of gravity at these
scales. Intending to provide a basic introduction to this fast developing area,
we present a general overview of theories with large extra dimensions. We
center our discussion on models for neutrino masses, high dimensional
extensions of the Standard Model and gauge coupling unification. We discuss the
recently proposed technic of splitting fermion wave functions on a tick brane
which may solve the problem of a fast proton decay and produce fermion mass
hierarchies without invoking extra global symmetries. Randall-Sundrum model and
some current trends are also commented.
