Marine environments are characterized by broad fluctuations of environmental conditions that can be strong stress inducers in macroalgal populations, including extreme temperatures, rapid salinity and nutrient changes, desiccation, intense sunlight, and others. These factors are extremely important for the geographical distribution of macroalgae and also can be responsible for several physiological responses of these organisms. the current book conclude that, the effects of various stresses on algae led to change in gene expression and its related to of secondary metabolite production which have different biological activities as antioxidant, anticancer and antimicrobial activities.