Different theorists have different categorizations and conceptualizations of defence mechanisms. Large reviews of theories of defence mechanisms are available from Paulhus, Fridhandler and Hayes (1997) and Cramer (1991). The Journal of Personality published a special issue on defence mechanisms (1998).

In the first definitive book on defence mechanisms, The Ego and the Mechanisms of Defence (1936), Anna Freud enumerated the ten defence mechanisms that appear in the works of her father, Sigmund Freud: repression, regression, reaction formation, isolation, undoing, projection, introjection, turning against one's own person, reversal into the opposite, and sublimation or displacement.

Sigmund Freud posited that defence mechanisms work by distorting id impulses into acceptable forms, or by unconscious or conscious blockage of these impulses. Anna Freud considered defense mechanisms as intellectual and motor automatisms of various degrees of complexity, that arose in the process of involuntary and voluntary learning.

Anna Freud introduced the concept of signal anxiety; she stated that it was "not directly a conflicted instinctual tension but a signal occurring in the ego of an anticipated instinctual tension". The signalling function of anxiety was thus seen as crucial, and biologically adapted to warn the organism of danger or a threat to its equilibrium. The anxiety is felt as an increase in bodily or mental tension, and the signal that the organism receives in this way allows for the possibility of taking defensive action regarding the perceived danger.

Both Freuds studied defence mechanisms, but Anna spent more of her time and research on five main mechanisms: repression, regression, projection, reaction formation, and sublimation. All defence mechanisms are responses to anxiety and how the consciousness and unconscious manage the stress of a social situation.

Repression: when a feeling is hidden and forced from the consciousness to the unconscious because it is seen as socially unacceptable
Regression: falling back into an early state of mental/physical development seen as "less demanding and safer"
Projection: possessing a feeling that is deemed as socially unacceptable and instead of facing it, that feeling or "unconscious urge" is seen in the actions of other people
Reaction formation: acting the opposite way that the unconscious instructs a person to behave, "often exaggerated and obsessive". For example, if a wife is infatuated with a man who is not her husband, reaction formation may cause her to – rather than cheat – become obsessed with showing her husband signs of love and affection.
Sublimation: seen as the most acceptable of the mechanisms, an expression of anxiety in socially acceptable ways
Otto F. Kernberg (1967) developed a theory of borderline personality organization of which one consequence may be borderline personality disorder. His theory is based on ego psychological object relations theory. Borderline personality organization develops when the child cannot integrate helpful and harmful mental objects together. Kernberg views the use of primitive defence mechanisms as central to this personality organization. Primitive psychological defences are projection, denial, dissociation or splitting and they are called borderline defence mechanisms. Also, devaluation and projective identification are seen as borderline defences.

In George Eman Vaillant's (1977) categorization, defences form a continuum related to their psychoanalytical developmental level.They are classified into pathological, immature, neurotic and "mature" defences.

Robert Plutchik's (1979) theory views defences as derivatives of basic emotions, which in turn relate to particular diagnostic structures. According to his theory, reaction formation relates to joy (and manic features), denial relates to acceptance (and histrionic features), repression to fear (and passivity), regression to surprise (and borderline traits), compensation to sadness (and depression), projection to disgust (and paranoia), displacement to anger (and hostility) and intellectualization to anticipation (and obsessionality).

The Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) published by the American Psychiatric Association (1994) includes a tentative diagnostic axis for defence mechanisms. This classification is largely based on Vaillant's hierarchical view of defences, but has some modifications. Examples include: denial, fantasy, rationalization, regression, isolation, projection, and displacement.
What are the defense mechanisms?
There are multiple definitions of defense mechanism based on different theories. Generally, defense mechanisms are human responses responses to anxiety and how the consciousness and unconscious manage the stress of a social situation. Some of the most common defense mechanisms include:

Repression: forcing a feeling from the consciousness to the unconscious because it is seen as socially unacceptable

Regression: falling back into an early state of mental/physical development seen as less demanding and safer

Projection: possessing a feeling that is deemed as socially unacceptable and instead of facing it, that feeling or "unconscious urge" is seen in the actions of other people

Reaction formation: acting the opposite way that the unconscious instructs a person to behave, "often exaggerated and obsessive". For example, if a wife is infatuated with a man who is not her husband, reaction formation may cause her to – rather than cheat – become obsessed with showing her husband signs of love and affection.

Sublimation: seen as the most acceptable of the mechanisms, an expression of anxiety in socially acceptable ways