  Given $n$ distinct points $t_1,\ldots,t_n$ on the unit circle $\T$ and
equally many target values $\f_1,\ldots,\f_n\in\T$, we describe all Blaschke
products $f$ of degree at most $n-1$ such that $f(t_i)=\f_i$ for
$i=1,\ldots,n$. We also describe the cases where degree $n-1$ is the minimal
possible.
