  The theory of perturbation of Friedman-Robertson-Walker (FRW) cosmology is
analysed exclusively in terms of observable quantities. Although this can be a
very complete and general procedure we limit our presentation here to the case
of irrotational perturbations for simplicity. We show that the electric part of
Weyl conformal tensor {\bf $E$} and the shear {\bf $\Sigma$} constitute the two
basic perturbed variables in terms of which all remaining observable quantities
can be described. Einstein\rq s equations of General Relativity reduce to a
closed set of dynamical system for {\bf $E$} and {\bf $\Sigma$}. The basis for
a gauge-invariant Hamiltonian treatment of the Perturbation Theory in the FRW
background is then set up.
