**Abstract**

This chapter focuses on the essential oils and volatile fractions of seaweed. It includes an introduction to the essentials and volatile fractions and the main chemical classes found. This part is completed by a presentation of the fundamental aspects of biodiversity and the chemodiversity of the marine environment followed by the taxonomy and systematics of marine macroalgae. The heart of this chapter concerns the chemistry of volatile products extracted from marine algae. It reports the specificities of the marine natural products chemistry in comparison to that of terrestrial organisms. The description of volatile compounds in seaweed is divided into two parts, the first reports the common compounds identified in main volatile fractions and the second cover the specific volatile components. These include C11 hydrocarbons, sulfur compounds, and halogenated hydrocarbons. These latter are playing a very important role in communication and chemical defense. The last part includes aspects of chemical ecology and biological activities of volatile products.

**Keywords:** essential oils, marine algae, C11 hydrocarbons, sulfur compounds, halogenated sesquiterpenes, chemical ecology, biological activities
