The PhD thesis describes in detail the procedures, methods and techniques we elaborated in order to achieve a full phytochemical analysis of the organic extract of the Aegean brown alga Taonia atomaria. It also includes the pharmacological evaluation of some of the isolated secondary metabolites. We worked on two separate extracts (29.5g and 180.4g) from two different (chronologically) collections of the organism, both of them from the island of Serifos. The choice to work on two different extracts came from the need to increase the quantities of the isolated metabolites both for receiving more reliable NMR spectra and for having the capability to perform more and more reliable pharmacological tests. The Theoretical Part of the Thesis is a review on the phytochemistry of the Dictyotales - for the first time – including elements of pharmacognosy, chemical ecology and organic synthesis of their isolated metabolites. The Experimental Part A (Chromatographic Part) includes all the chromatographic procedures and techniques we used (GCC, VCC, HPLC) for the separations and the final purifications of the metabolites. Before we proceed to the Chromatographic Part we introduce the whole combined network of the HPLC ‘s in seven charts including details like the nature of the stationary phases, the solvent systems, the codification of the peaks and their poolings. The Experimental Part B contains the Structure Elucidation of the isolated metabolites including the worked NMR spectra (one and two dimensional), and the MS spectra as well as the detailed analysis we performed in order to define the fine structure of the molecules (relative stereochemistry). The Experimental Part C contains the pharmacological evaluation of some of the isolated metabolites including the anti-oxidant, anti-inflammatory, cytostatic and the anti-fouling activity. We isolated 48 secondary metabolites all of them (except for the Metabolites 39, 40, 41, 42 and 43) belonging to the chemical family of Meroditerpenoids. The isolated Meroditerpenoids can be further distinguished into 4 different subfamilies. The first one (LmD, Linear mero-Diterpenoids) includes the Metabolites 1-22, which are all derivatives of saragaquinone (Metabolite 1) or sargaquinole (Sty.7, Theoretical Part, a metabolite that we didn’t isolate from T. atomaria), with modifications on the prenyl-chain or/and on the benzoquinonic or hydroquinolic ring (geometrical isomerization always on the first prenyl-group, hydroxylations / acetylations / methoxylations on the prenyl-chain, fatty-acid acetylations of the phenyl-hydroxyl-group of the aromatic ring. Eighteen of these metabolites are new chemical structures (Metabolites 2-8, 10, 11 13-22) while two of them are new natural products (Metabolites 9, 12). Several of these linear metabolites were isolated as inseparable mixtures of 2Z/2E isomers with the 2Z isomer being in all cases the minor constituent (Metabolites 3/4, 5/6, 7/8, 9/10, 13/14, 15/16, 18/19) but we managed to elucidate their structures and to make the chemical assignments (in all cases for the major 2E). X The second sub-family (CmD) contains five chromenol/chromanol meroditerpenoids (Metabolites 23-27), two of which are new chemical structures (Metabolites 24 and 25). The third sub-family (ACmD) includes the cyclised meroditerpenoids which in turn is divided into three further categories, depending on the way the aromatic-part of the molecule is connected to the prenyl-part. The first category (ACmD-I) includes the benzopyranic cyclised meroditerpenoids, that is the molecules that contain a benzopyranic ring for connecting the two parts of it. We isolated 5 metabolites of this category (Metabolites 28-32), two of them being new chemical structures. The second category (ACmD-II) includes the benzofuranic cyclised meroditerpenoids which contain a benzofuranic ring for connecting the aromatic and the prenyl-parts of the molecules. ..