Kierkegaard and Nietzsche
The first philosophers considered fundamental to the existentialist movement were Søren Kierkegaard and Friedrich Nietzsche, though neither used the term "existentialism" and it is unclear whether they would have supported the existentialism of the 20th century. Their focus was on human experience, rather than the objective truths of math and science that are too detached or observational to truly get at human experience. Like Pascal, they were interested in people's concealment of the meaninglessness of life and the use of diversion to escape from boredom. But Pascal did not consider the role of making free choices, particularly regarding fundamental values and beliefs: such choices change the nature and identity of the chooser, in the view of Kierkegaard and Nietzsche. Kierkegaard's knight of faith and Nietzsche's Übermensch are examples of those who define the nature of their own existence. Great individuals invent their own values and create the very terms under which they excel.
Heidegger and the German existentialists
One of the first German existentialists was Karl Jaspers, who recognized the importance of Kierkegaard and Nietzsche and attempted to build an "Existenz" philosophy around the two. Heidegger, who was influenced by Jaspers and the phenomenologist Edmund Husserl, wrote his most influential work Being and Time which postulates Dasein (dah-zine), translated as, all at once, "being here", "being there", and "being-in-the-world"—a being that is constituted by its temporality, illuminates and interprets the meaning of being in time. Dasein is sometimes considered the human subject, but Heidegger denied the Cartesian dualism of subject-object/mind-body. [paragraph needs citations and clarifications]
Although existentialists view Heidegger to be an important philosopher in the movement, he vehemently denied being an existentialist in the Sartrean sense, in his "Letter on Humanism".
Sartre, Camus, and the French existentialists
Jean-Paul Sartre is perhaps the most well-known existentialist and is one of the few to have accepted being called an "existentialist". Sartre developed his version of existentialist philosophy under the influence of Husserl and Heidegger. Being and Nothingness is perhaps his most important work about existentialism. Sartre was also talented in his ability to espouse his ideas in different media, including philosophical essays, lectures, novels, plays, and the theater. No Exit and Nausea are two of his celebrated works. In the 1960s, he attempted to reconcile existentialism and Marxism in his work Critique of Dialectical Reason.
Albert Camus was a friend of Sartre, until their falling-out, and wrote several works with existential themes including The Rebel, The Stranger, The Myth of Sisyphus, and Summer in Algiers. Camus, like many others, rejected the existentialist label, and considered his works to be concerned with man facing the absurd. In The Myth of Sisyphus, Camus uses the analogy of the Greek myth to demonstrate the futility of existence. In the myth, Sisyphus is condemned for eternity to roll a rock up a hill, but when he reaches the summit, the rock will roll to the bottom again. Camus believes that this existence is pointless but that Sisyphus ultimately finds meaning and purpose in his task, simply by continually applying himself to it.