  Numerical Relativity is a multidisciplinary field including relativity,
magneto-hydrodynamics, astrophysics and computational methods, among others,
with the aim of solving numerically highly-dynamical, strong-gravity scenarios
where no other approximations are available. Here we describe some of the
foundations of the field, starting from the covariant Einstein equations and
how to write them as a well-posed system of evolution equations, discussing the
different formalisms, coordinate conditions and numerical methods commonly
employed nowadays for the modeling of gravitational wave sources.
