There were two main periods in his career. The first period was defined by his work Being and Nothingness. He believed in the fundamental freedom of human beings and reflected on what he saw as the unbearable nature of that freedom.
In the second major period in his career, Sartre was known as a politically engaged intellectual. He embraced Communism, though he never officially joined the Communist party. Sartre spent much of his life attempting to reconcile his existentialist ideas, which claimed that one must self-determine one's existence, with Communist principles, which taught that socioeconomic forces beyond one's control play a critical role in determining the course of one's life.