  We study point and higher symmetries for the hydrodynamic-type systems with
two independent variables $t$ and $x$ with and without explicit dependence of
the equations on $t,x$. We consider those systems which possess an
infinite-dimensional group of the hydrodynamic symmetries, establish existence
conditions for this property and, using it, derive linearizing transformations
for these systems. The recursion operators for symmetries are obtained and used
for constructing infinite series of exact solutions of the studied equations.
Higher symmetries, i.e. the Lie-Backlund transformation groups, are also
studied and the interrelation between the existence conditions for higher
symmetries and recursion operators is established. More complete results are
obtained for two-component systems, though $n$-component systems are also
studied. In particular, we consider Hamiltonian and semi-Hamiltonian systems.
