In the four parts of the book, Barrett explains in brief the philosophical tradition to which existentialism was a reaction, and then the main concepts of existential thought. In the first part, "The Present Age", Barrett shows the impact that existentialism has had on culture even without being a widely known philosophical school of thought. In the second part, "Sources of Existentialism", Barrett traces the development of philosophy as it pertains to being, ontology, and metaphysics. He shows the contrast between existentialist thought and other forms of philosophy. In part three, "The Existentialists", Barrett introduces Søren Kierkegaard, Friedrich Nietzsche, Martin Heidegger, and Jean-Paul Sartre, the four main philosophical thinkers, along with their primary thoughts and terminology. The discussion of each philosopher serves as a road map for those seeking an entre into the primary works of each, which can be dense with unique terminology. The material in each of these four sections summarizes the main points each philosopher contributed to existentialism. The reactions of one philosopher to another is also explored. The philosophers are also situated in the larger history of philosophical investigations outside of existentialism itself. Barrett concentrates on these main philosophers and avoids an in-depth discussion (although he does mention some) of the many of the existentialist artists and writers.
In part four, "Integral vs Rational Man, Barrett applies existentialist thought to the world of the late 1950s, during the Cold War. The work includes two appendices, "Negation, Finitude, and the Nature of Man", which reprints a paper presented by Barrett in 1957, and "Existence and Analytic Philosophers", a discussion of existentialism and analytic philosophy.