Integrable structures in matrix models and physics of 2d-gravity

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.