This paper examines the principal ideas from Jacques Lacan's psychoanalytic theory of psychosis. According to Lacan's theory in the 1950s, the central organizing element of symbolically organized mental life, the Name-of-the-Father, is missing in psychosis. That theory changes with later conceptual developments in Lacan's work that focus on the incompleteness of symbolic functioning. This connects with how, in his works from the late 1960s and the 1970s, Lacan embraces the idea of a fundamental non-rapport and symbolic non-existence at the basis of mental life. In a second step, the paper explores what the Lacanian model of psychosis implies with regard to ethical positioning, addressing the unconscious, handling transference, and crisis and stability in psychosis. A clinical case discussion focuses on a yearlong therapeutic trajectory with a young man with Down's syndrome who suffered from psychotic experiences.