  We study abstract Ces\`aro spaces $CX$, which may be regarded as
generalizations of Ces\`aro sequence spaces $ces_p$ and Ces\`aro function
spaces $Ces_p(I)$ on $I = [0,1]$ or $I = [0,\infty)$, and also as the
description of optimal domain from which Ces\`aro operator acts to $X$. We find
the dual of such spaces in a very general situation. What is however even more
important, we do it in the simplest possible way. Our proofs are more
elementary than the known ones for $ces_p$ and $Ces_p(I)$. This is the point
how our paper should be seen, i.e. not as generalization of known results, but
rather like grasping and exhibiting the general nature of the problem, which is
not so easy visible in the previous publications. Our results show also an
interesting phenomenon that there is a big difference between duality in the
cases of finite and infinite interval.
