Displacing fossil fuels and their derivatives with renewables and increasing sustainable food and materials production are among the major challenges facing the world in the coming decades. A possible, sustainable direction for addressing these challenges is biorefinery. Plants and microalgae are possible feedstocks; however, concerns over net energy balance, potable water use, environmental hazards, and uncertainty in the processing technologies question their actual potential to meet the anticipated food, feed, materials, and energy challenges in a sustainable way. Alternative sustainable sources for biorefineries are offshore-produced macroalgae. However, implementation of the offshore biorefineries requires detailed analysis of their technological, economic, and environmental performance. In this chapter, the basic principles of marine biorefineries design are shown. The methods to integrate thermodynamic efficiency, investment and environmental aspects are discussed. The performance improvements by development of new cultivation and dewatering methods that fit macroalgae physiology and development of new conversion methods that address macroalgae’s unique chemical composition are shown.