The significance of Winnicott's work for the theory and practice of adult analysis is examined. Winnicott's most valuable contribution was to discern important new clinical phenomena. This is a crucial aspect of theory development, but not the same as explicating new theory, and partly accounts for the difficulty integrating his ideas with traditional psychoanalytic theory. Winnicott left the theoretical revisions possibly necessitated by his discoveries for others to accomplish, and some of this new work is discussed and synthesized. The nature of Winnicott's discoveries, especially the phenomenological field of transitional or intermediate experience, is elusive to define, and is also related to his clinical stance and his attitudes to theory for the purposes of clinical work. His work demonstrates the limits of theorizing and of our theories, and also an attitude toward theory that facilitates clinical and theoretical progress. He also provides conceptual language to discuss what is often called the art of psychoanalysis. Both the value and limitations of studying Winnicott are considered.