  We construct Lagrange interpolating polynomials for a set of points and
values belonging to the algebra of real quaternions $H\simeq R_{0,2}$, or to
the real Clifford algebra $R_{0,3}$. In the quaternionic case, the approach by
means of Lagrange polynomials is new, and gives a complete solution of the
interpolation problem. In the case of $R_{0,3}$, such a problem is dealt with
here for the first time. Elements of the recent theory of slice regular
functions are used. Leaving apart the classical cases $R_{0,0}\simeq R$,
$R_{0,1}\simeq C$ and the trivial case $R_{1,0}\simeq R\oplus R$, the
interpolation problem on Clifford algebras $R_{p,q}$ with
$(p,q)\neq(0,2),(0,3)$ seems to have some intrinsic difficulties.
