Preliminaries on Hypercomplex Analysis.- Approximation of Continuous Functions.- Approximation by Bernstein and Convolution Type Operators.- Approximation of Slice Regular Functions in Compact Sets.- Overconvergence, Equiconvergence and Universality Properties.- Inequalities for Quaternionic Polynomials.- Approximation of nullsolutions of generalized Cauchy-Riemann operators.