ABSTRACT. We construct a system of CR-invariants of a manifold generated by projective invariants of the tangent quadric. We present a description of the group of projective diffeomorphisrns of a quartic. We also estimate the degree of a rational mapping of a quartic. The description problem for subgroups of a Cremona group of bounded degree is posed.w Introduction Consider the germ M of a smooth real surface at the origin of the complex linear space of dimension N _> 2. Then in some neighborhood of the origin the germ M may be given by the system of equationsIf the vectors {grad~a(0), p = 1,..., k} are C-linearly independent, a complex linear transformation takes the equations of M to the formhere F is a smooth mapping of a neighborhood of the origin in C n x R k into R k , and F and dF vanish at Z = 0, ReW = 0. Among such manifolds one meets (k, n)-quadrics, i.e., surfaces of the formwhere ( i.e., the surface and the quadric are tangent up to the third order. In this case the (k, n)-quadric, which we denote by Q, is called the tangent quadric to the surface M. Note that the dimension of M is k + 2n, Im W = 0 is the equation of the tangent plane at the origin, W = 0 is the equation of the complex tangent plane, i.e., n is the dimension of the complex tangent plane; and (Z, Z) is the Levi form of the surface M at the origin. Let us recall some information on tangent quartics.