Neidan (內丹, "inner alchemy") is the tradition of Taoist cultivation that reached its fullest development in the Tang, Song, and Ming dynasties, in texts like the Cantong Qi and the Awakening to Reality (Wuzhen Pian). It takes the language and conceptual framework of outer alchemy — the laboratory transformation of base metals into gold, the creation of elixirs of immortality — and maps it onto the processes happening inside the practitioner's own body and mind. The three primary substances of inner alchemy correspond to the three "treasures" of Taoist physiology: jing (essence, associated with the lower cinnabar field in the abdomen and the body's basic vitality), qi (vital breath, associated with the middle cinnabar field in the chest), and shen (spirit, associated with the upper cinnabar field in the head). The alchemical work consists of cultivating these three energies, refining each progressively: through specific practices, jing is converted into qi, qi is refined into shen, and shen is finally returned to the Void — the undifferentiated ground from which all phenomena arise. This process, called "reversal" (ni), moves against the ordinary direction of biological entropy and is considered the path to genuine spiritual liberation.

The practical techniques of neidan include breath retention and regulation, visualization of energy moving through specific channels and fields in the body, cultivation of deep meditative absorption, dietary regulation, sexual energy management, and the use of specific postures and hand gestures that facilitate energetic processes. The foundational instruction — to turn attention inward and observe the arising and passing of internal phenomena with sustained, non-grasping awareness — shares much with Buddhist meditation. The difference is in the specific model of what is happening: where Buddhist practice generally aims at the direct seeing of impermanence and non-self, neidan is working with what it considers to be real subtle-body processes that can be strengthened and refined. The phenomenology, however, overlaps substantially — practitioners in both traditions describe experiences of expanded awareness, dissolution of the sense of physical boundary, luminosity, stillness, and what is variously called emptiness, open awareness, or the Tao. Contemporary practitioners of neidan and the related moving practices of qigong and tai chi report that the system is teachable, transmittable, and genuinely transformative when practiced with sustained attention and appropriate guidance — regardless of how one ultimately interprets the metaphysical framework.