The present invention relates to substituted heterocyclic compounds of Formula I: or pharmaceutically acceptable salts or N-oxides or quaternary ammonium salts thereof wherein constituent members are provided hereinwith, as well as their compositions and methods of use, which are histamine H4 receptor inhibitors/antagonists useful in the treatment of histamine H4 receptor-associated conditions or diseases or disorders including, for example, inflammatory diseases or disorders, pruritus, and pain.