Collaborative tagging is a new content sharing and organization trend, mainly diffused over the Web, which has attracted growing attention during the last years. It refers to the process by which many users add metadata in the form of keywords to shared content. Today many different collaborative tagging systems are available on the Web, enabling users to add descriptive keywords to different types of Internet resources (web pages, photos, videos, etc.). The great number of advantages offered by the availability of collaboratively tagged resources in terms of their organization and shared information is underlined by their growing adoption, also in non-technical communities of users. In spite of this, analyzing the current structure and usage patterns of collaborative tagging systems, we can discover many important aspects which still need to be improved so as to bring tagging systems to their full potential. In particular, problems related to synonymy, polysemy, different lexical forms, different spellings and misspelling errors, but also the lack of accurancy caused by different levels of precision and distinct kinds of tag-to-resource association represent a great limit, causing inconsistencies among the terms used in the tagging process and thus reducing the efficiency of content search and the effectiveness of the tag space structuring and organization.