In the classification of the plant kingdom, the microalgae occupy a particular place from their geographical localization, their behavior with respect to the environmental constraints but also from their morphology, their physiology and their biochemistry. These various aspects make their cellular machineries very flexible and able to produce many organic molecules with various biological activities such as proteins, polysaccharides, pigments and lipids of interest. With regard to the lipids, microalgae are able to produce many varieties and great amounts of fatty acids, especially of polyunsaturated fatty acids, making them candidates of interest within the framework of the production and exploitation of lipids in various sectors of industry and health. These fatty acids present in the lipids produced by microalgae can be used alike manner as those resulting from marine sources of animal origin such as fish in the field from health. The aims of this chapter are to present the biochemical diversity of the lipids and the fatty acids contained in various species of microalgae of marine and freshwater environments. Then, this review will give examples of how these plant resources can represent an alternative to the animal ones, particularly fish, in the area of the prevention of pathologies such as cardiovascular diseases and cancers.