The semantic gap is the main problem of content based multimedia retrieval. This refers to the extraction of the semantic content of multimedia documents, the understanding of user information needs and requests, as well as to the matching between the two. In this chapter we focus on the analysis of multimedia documents for the extraction of their semantic content. Our approach is based on fuzzy algebra, as well as fuzzy ontological information. We start by outlining the methodologies that may lead to the creation of a semantic index; these methodologies are integrated in a video annotating environment. Based on the semantic index, we then explain how multimedia content may be analyzed for the extraction of semantic information in the form of thematic categorization. The latter relies on stored knowledge and a fuzzy hierarchical clustering algorithm that uses a similarity measure that is based on the notion of context.