We discuss quadratic transformations for orthogonal polynomials in one and
two variables. In the one-variable case we list many (or all) quadratic
transformations between families in the Askey scheme or q-Askey scheme. In
the two-variable case we focus, after some generalities, on the polynomials
associated with root system BC2, i.e., BC2-type Jacobi polynomials if
q=1 and Koornwinder polynomials in two variables in the q-case.Comment: v2: minor corrections, 24 page