Drug effects on consciousness are powerful probes of how physical processes in the body are connected to conscious experience. Drugs that alter consciousness – producing arousal, sedation, sleep, anesthesia, analgesia, euphoria, amnesia, hallucinations, or psychedelic-like intensification of perceptions, thoughts, and feelings – have been identified as interacting in various ways with cellular and molecular processes within the nervous system. While the focus has thus far been on synaptic connections between neurons, there is likely to be much more going on in the interaction of drugs with living systems that has not yet been identified. Psychedelic drugs, plants, and fungi are associated with perhaps the most interesting and profound of the drug-induced alterations of consciousness. Contemporary investigations of psychedelics include renewed interest in the transformational therapeutic potential of these substances, and may also lead to new insights into the nature of consciousness.