  A review of the appearence of integrable structures in the matrix model
description of $2d$-gravity is presented. Most of ideas are demonstrated at the
technically simple but ideologically important examples. Matrix models are
considered as a sort of "effective" description of continuum $2d$ field theory
formulation. The main physical role in such description is played by the
Virasoro-$W$ constraints which can be interpreted as a certain unitarity or
factorization constraints. Bith discrete and continuum (Generalized Kontsevich)
models are formulated as the solutions to those discrete (continuous)
Virasoro-$W$ constraints. Their integrability properties are proven using
mostly the determinant technique highly related to the representation in terms
of free fields. The paper also contains some new observations connected to
formulation of more general than GKM solutions and deeper understanding of
their relation to $2d$ gravity.
