Microalgae are eukaryotic microorganisms that photosynthesize to produce biomass and many other useful products. Certain microalgae are used as human food supplements, feed, biofuel, cosmetics, and pharmaceuticals. In order to grow and tap the potentials of microalgae, four distinct modes of growth are recognized: photoautotrophy, heterotrophy, mixotrophy, and photoheterotrophy. There are two basic systems for microalgal culture: open systems where the culture is exposed to the atmosphere and closed systems, commonly called photobioreactors, in which the microalga has little or no exposition to the atmosphere. Once the biotechnological interest of many of these autotrophic microorganisms has been recognized, the next step is the development of bioprocesses that link scientific discoveries to business needs. The design and optimization of culture strategies and bioreactors suitable for cultivating these microorganisms is an essential part of this approach. In this chapter, the main characteristics of microalgae, their classification, the factors that influence their growth, the types of cultivation, the different cultivation systems and the potential applications of these photosynthetic microorganisms are described.