  We present explicit Pieri formulas for Macdonald's spherical functions (or
generalized Hall-Littlewood polynomials associated with root systems) and their
$q$-deformation the Macdonald polynomials. For the root systems of type $A$,
our Pieri formulas recover the well-known Pieri formulas for the
Hall-Littlewood and Macdonald symmetric functions due to Morris and Macdonald
as special cases.
