Existentialism is a philosophical movement that arguably began in the works of the German Idealists in the early Nineteenth Century. These thinkers shared a philosophical concern over the nature of the self that then began to develop in the works of the likes of Kierkegaard into something that became more recognisable as existentialism.
With its focus on the individual as autonomous and self-defining, existentialism addressed issues like subjectivity, self-consciousness and living an 'authentic' existence that was the result of making real choices. As Sartre put it in the Twentieth Century, 'man first of all exists, and defines himself afterwards'.