  A framework is developed in which one can write down the constraint equations
on a three--dimensional hypersurface of arbitrary signature. It is then applied
to isolated and dynamical horizons. The derived equations can be used to
extract physicaly relevant quantities describing the horizon irrespective to
whether it is isolated (null) or dynamical at a given instant of time.
Furthermore, small perturbation of isolated horizons are considered, and
finally a family of axially--symmetric exact solution of the constraint
equations on a dynamical horizon is presented.
