The term autosuggestion is used for positive or negative physical symptoms explained by the thoughts and beliefs of a person. For example, some will experience more pain when they think it will hurt. Headaches sometimes go away after taking a painkiller, but before the painkiller could actually start acting on its own. Related to this is the placebo-effect.

This influence of the mind on the body can be used in a positive way to improve the way a person feels (mentally or physically).

Autosuggestion (or the related autogenic training) is a process by which an individual trains the subconscious mind to believe something, or systematically schematizes the person's own mental associations, usually for a given purpose.

This is accomplished through self-hypnosis methods or repetitive, constant self-affirmations, and may be seen as a form of self-induced brainwashing. The acceptance of autosuggestion may be quickened through mental visualization of that which the individual would like to believe. Its success is typically correlated with the consistency of its use and the length of time over which it is used. Autosuggestion can be seen as an aspect of prayer, self-exhorting "pep talks", meditation, and other similar activities. A trivial example of self-improvement by autosuggestion is the New Year's resolution, especially if it is followed up by systematic attention to the resolution.

Autosuggestion is most commonly accomplished by presenting (either through caressing or bombarding) one's mind with repetitive thoughts (negative or positive), until those thoughts become internalized. Practitioners typically hope to transmute thoughts into beliefs, and even into actualities. Visualizing the manifestations of a belief, verbally affirming it, and thinking it using one's "internal voice", are typical means of influencing one's mind via repetitive autosuggestion. Autosuggestion is normally thought of as a deliberate tool, but it can also refer to an unintentional process.

The French psychologist Émile Coué wrote extensively on the theory and practice of autosuggestion.

Applications of deliberate autosuggestion are intended to change: the way one believes, perceives, or thinks; one's acts; or the way one is composed physically or physiologically. An example might be individuals reading nightly aloud a statement they have written describing how they would like to be, then repeating the statement in their mind until they fall asleep. People have attributed changes to such a nightly routine or similar employment of autosuggestion, for example, increased confidence, the conquering of life-long fears, heightened mental faculties (e.g., ability to calculate mathematics or read at a quicker rate), eradication of diseases or infections from one's body, and even improved eyesight and growing taller. It is not uncommon to hear people claim that they have been able to get rid of warts on their hands, simply by making a point of saying, "There go my warts!" every time they saw a garbage truck or a trashcan, but it is not clear whether such anecdotal reports should be taken as evidence of the power of autosuggestion. The ability to fight sicknesses and infections, as well as many other things, shows that it may be a form of a placebo. Making yourself "believe" the body is curing the sickness by itself may affect what your cells and body do, although this hasn't been conclusively tested.

The same type of effect that deliberate autosuggestion may achieve can also be seen in individuals not consciously trying to program themselves through autosuggestion. The dominant thoughts that occupy a person's conscious mind, if constantly present over an extended period of time, may have the effect of training that person's subconscious mind to organize that individual's beliefs according to those thoughts. In this sense, the mechanisms of pathological fixations and obsessions to some extent resemble the process of autosuggestion.

Autosuggestion is differentiated from brainwashing in that the suggestions given during the sessions originate with the individual, rather than originating with suggestions from others.

Johannes Schultz developed this theory as Autogenic training.