By analysing the current structure and the usage patterns of collaborative tagging systems, we can find out many important aspects which still need to be improved. Problems related to synonymy, polysemy, different lexical forms, mispelling errors or alternate spellings, different levels of precision and different kinds of tag-to-resource association cause inconsistencies and reduce the efficiency of content search and the effectiveness of the tag space structuring and organization. They are mainly caused by the lack of semantic information inclusion in the tagging process. We propose a new way to describe resources: the semantic tagging. It allows user to state semantic assertions: each of them expresses a defined characteristic of a resource associating it with a concept. We present SemKey, a semantic collaborative tagging system, describing its global architecture and functioning along with the most relevant organizational issues faced. We explore the adequacy of the support offered by the entries of Wikipedia and WordNet in order to access to and reference concepts.