The dominant figure of second-generation Frankfurt School Critical Theory and the most influential German philosopher of the postwar era. Habermas rebuilt Critical Theory on foundations more hopeful than those of Adorno and Horkheimer, insisting against their near-despair that modern rationality is not simply instrumental domination but also contains the emancipatory potential of communicative action — the cooperative reaching of understanding through argument in which the only force is the force of the better reason.
The Structural Transformation of the Public Sphere (1962) gave historians and political theorists a concept they're still using: the "public sphere" of coffeehouses, journals, and associations that emerged in 18th-century Europe as a space where private persons reasoned together about matters of common concern. His mature masterwork, The Theory of Communicative Action (1981), distinguishes the "lifeworld" (the background of shared meanings and practices within which we communicate) from the "system" (money and administrative power) and warns that the system increasingly colonizes the lifeworld — reducing citizens to consumers and clients.
Between Facts and Norms (1992) brought Habermas closer to liberal political philosophy, treating democratic legitimacy as grounded in procedures of inclusive deliberation among free and equal citizens. The affinity with John Rawls's later work is close; the two held a famous exchange in the 1990s. Critics on the left (including old Frankfurt comrades) accused him of domesticating Critical Theory; critics on the right accused him of smuggling in a too-thick Enlightenment optimism. He has kept writing into his nineties.