  The optimal Orlicz target space is exhibited for embeddings of
fractional-order Orlicz-Sobolev spaces in $\mathbb R^n$. An improved embedding
with an Orlicz-Lorentz target space, which is optimal in the broader class of
all rearrangement-invariant spaces, is also established. Both spaces of order
$s\in (0,1)$, and higher-order spaces are considered. Related Hardy type
inequalities are proposed as well. An extension theorem is proved, that enables
us to derive embeddings for spaces defined in Lipschitz domains. Necessary and
sufficient conditions for the compactness of fractional Orlicz-Sobolev
embeddings are provided.
