The biodiversity of phytoplankton makes them an attractive source of microalgal species and strains that could be a feedstock for the production of a variety of biomolecules. In this chapter, we focus on the molecular diversity of fatty acids and glycerolipid classes containing fatty acid esters, including phosphoglycerolipids, galactoglycerolipids, and triacylglycerols. In the first part, we describe the general structures of the main fatty acids and glycerolipids found in photosynthetic cells. We then summarize the general metabolic pathways in the different taxa representing the biodiverse phytoplankton. Finally, we detail some general questions that should be addressed to exploit phytoplankton glycerolipids in applications ranging from feed, food, and biomedicine to green chemistry and biofuels.