The idea that states should provide a means-tested guaranteed minimum income for citizens who are unable to meet their basic needs is widely shared and has been a central component in the evolution of social citizenship rights in existing welfare states. However, an increasing number of activists and scholars defend the more radical option of establishing a universal basic income, that is, an unconditional income paid to all members of society on an individual basis without any means test or work requirement. Indeed, some political philosophers have argued that basic income is one of the most important reforms in the development of a just and democratic society, comparable to other milestones in the history of citizenship rights, such as universal suffrage or even the abolishment of slavery. Basic income or similar ideas, such as a basic capital or a negative income tax, have been advanced in many versions since the 18th century in different parts of the world and under a great variety of names. However, while these were previously often isolated and disconnected initiatives, basic income has more recently become the object of an increasingly cumulative research effort to shed light on the many aspects of this idea. It has also inspired policy developments and given rise to experiments and pilot projects in several countries.