Jung developed an innovative theoretical-practical work with the Word Association Experiment, leading to his theory of complexes, a fundamental aspect in Analytical Psychology. In 1906, after reading Sigmund Freud’s Interpretation of Dreams and other of his works, he began a personal and professional relationship with Freud. Jung attributed to Freud the genius and boldness of bringing the work with the unconscious and its hidden forces in relation with neurotic symptoms. Jung agreed with many of his insights and hypotheses, but always questioned other aspects of Freud’s theory which seemed to be too reductionistic. Jung, with his immense intellectual capacity and profound insight, quickly became one of the central figures of psychoanalysis, Freud’s “crown prince”. However, Jung’s thought was already deeply developed when he met Freud and therefore disagreed with various aspects of psychoanalytic theory and practice, namely Freud’s attitude towards spirituality and religion as “nothing but” a collective neurosis and his monolithic attitude towards the centrality of sexuality to the psychic life. This led to a split between the two men in 1913. Jung left Freud and his functions within the psychoanalytical movement, leading to a profound personal and professional crisis which is documented and accessible in his Black Books and Red Book. This crisis was however profoundly transformative and creative, inviting Jung to explore his inner world and to develop ways of relating to the images that emerged on his inner journeys. During those years, central aspects of analytical technique were developed: the technique of active imagination, his theory of psychological types,and the theory of the collective unconscious and archetypes, among others. In his theory, it became clear that the inner life is clearly neglected and that this is the main cause of psychological and physical disorders. The extreme focus on the outer world and materialism in modernity, leads the individual to ignore his inner and spiritual (though not necessarily religious) dimensions. Jung considered this dimension as instinctive and an absolute necessity for the individual’s psychological balance. In a constant search for a better understanding of psychic reality, Jung later developed a deep interest in alchemy where he perceived a symbolic language that expressed the unconscious dynamics and relationship between Psyche and Soma. His profound conviction that Psyche and matter are interconnected, led him to his theory of synchronicity (developed with the physicist and Nobel Prize winner Wolfgang Pauli) and opened the door to the principles of modern Psychosomatics. Jung profoundly revolutionized the fields of psychology and psychotherapy, the relationship between psychology and spirituality and the applications of psychology to cultural phenomena. His work and thought have been expanded by successive generations of Jungian psychoanalysts who, inspired by his original work, have revised and deepened the concepts and applications of Analytical Psychology, making it a field of interdisciplinary inquiry.