I carry out research mostly in the field of approximation theory. In the recent years I have been particularly interested in establishing a precise and useful characterization of the rate of convergence of various approximation processes generated either by linear operators or by non-linear as best (weighted) approximation by algebraic or trigonometric polynomials. My research also includes interpolation, (generalized) Fourier expansions, orthogonal polynomials, quadrature formulae, multipliers, application of Fourier analysis in approximation theory, and functional analysis.