Marx’s defense of socialism and Mises’s defense of liberalism are among the most prominent attempts by modern economists to defend specific models of social organization. This chapter examines how Marx and Mises theorize needs and/or preferences and the significant influence of these theories on their defenses of socialism and liberalism. First, it shows that Marx theorizes needs formally, leading him to argue that basic needs cannot be distinguished from other needs. The chapter argues that his formal theories of needs weaken his arguments in favor of socialism. It moreover lays bare the deleterious influence of Marx’s formal theories of needs on his analysis of labor and its economic value. The chapter then argues that due to Mises’s merely formal theory of needs and preferences, the arguments he offers to defend liberalism are not convincing. It also shows that Mises’s formal theory of needs and preferences prevents him from explaining cooperation as a voluntary and spontaneous social phenomenon. In conclusion, the chapter maintains that less formal approaches to needs and preferences are imperative in light of the climate and ecological crises and allow us to examine the pros and cons of socialism and liberalism on a more rational basis.