  We introduce (integro-differential) harmonic maps into spheres, which are
defined as critical points of the Besov-Slobodeckij energy
$\int\limits_{\Omega}\int\limits_{\Omega}
\frac{|v(x)-v(y)|^{p_s}}{|x-y|^{n+sp_s}}\ dx\ dy$. For $p_s = 2$ these are the
classical fractional harmonic maps first considered by Da Lio and Riviere. For
$p_s \neq 2$ this is a new energy which has degenerate, non-local
Euler-Lagrange equations. For the critical case, $p_s = n/s$, we show Holder
continuity of these maps.
