  We study properties of solutions of $Q$-systems in the WZW fusion ring
obtained by the Kirillov-Reshetikhin modules. We make a conjecture about their
positivity and periodicity and give a proof of it in some cases. We also
construct a positive solution of the level $k$ restricted $Q$-system of
classical types in the fusion rings. As an application, we prove some
conjectures of Kirillov and Kuniba-Nakanishi-Suzuki on the level $k$ restricted
$Q$-systems.
