Freud developmental theories on psychosexual development were among the first attempts to bring psychology under the same scientific structure and methodology of medicine and biology. Part of this structure was in setting an objective course of "normal" human sexual development. Once establishing this baseline of development, Freud was able to identify various stages, and their associated consequences should a child experience trauma during that developmental period. The theory was based on a child being born "polymorphously perverse." This essentially means that a child is born a tabula rasa regarding which part of the body they can derive sexual pleasure from. Freud developmental theory focused on two pivotal elements of human nature, "sex" and "aggression." Socialization during the child's upbringing was what drove the development of the child's libido. The child's resultant libido would determine if they had developed anxiety or neuroses. These feelings of anxiety and neuroses came as a result of the child experiencing sexual frustration or repression during any one of the five stages of development. If the child's needs were met in full or at least adequately by healthy nurturing, they could advance into adulthood with a healthy personality.[1] 

The five stages of Freud psychosexual development theory are broken up as follows. Each stage contains an age range, stage name, an erogenous body part, and the associated consequence of fixation at that stage of development.

Stage I: 0-1 year, oral, mouth: Oral desire is the center of pleasure for the newborn baby. The earliest attachment of a baby is to the one that provides gratification to his oral needs, usually his mother. If the optimal amount of stimulation is not available, libidinal energy fixates on the oral mode of gratification. This would result in the child being orally aggressive or passive in a physical sense. In a more psychological vein, the child would develop an immature personality consisting of passiveness, gullibility, and manipulation.

Stage II: 1-3 years old, anal, bowel, and bladder: Toilet training is an especially sensitive task during this period of the child. The parents' desire for adequate performance shifts the libidinal energy from the oral to the anal area. The child faces increased chances to be reprimanded, to feel inadequate, and an increased ability to perceive a negative evaluation from a caretaker if he fails to perform in the right place at the right time. These experiences could lead a child to develop anal retentiveness or expulsiveness. The former being one of over-organization or neatness and the latter being one of carelessness and disorganization. 

Stage III: 3-6 years old, phallic, genitalia: This is perhaps the most controversial stage of Freud psychosexual development. This is the stage in which the child begins to experience pleasure associated with their genitalia. In this period of primitive sexual development, the child can establish the roots of fixation with the opposite sex parent, the Oedipus complex. However, it was soon after suggested that the Oedipus complex is not one of sexual envy, but of power envy.

Stage IV: 6 - 12 years old, latency, dormant sexual feelings: During this time, the libido is relatively repressed or sublimated. Freud did not identify any erogenous zone for this stage. The child now begins to act on their impulses indirectly by focusing on activities such as school, sports, and building relationships. Dysfunction at this stage results in the child's inability to form healthy relationships as an adult.

Stage V: 13-18 years old, genital, mature sexual feelings: The child's ego becomes fully developed during this stage, and they are subsequently seeking their independence. Their ability to create meaningful and lasting relationships is concrete, and their sexual desires and activity are healthy and consensual. If a child or young adult experiences dysfunction during this period, they will experience a loss or lacking the ability to develop meaningful, healthy relationships.

As described in the article "Freud's Clinical Theories Then and Now," Kupfersmid outlines the basic tenets of Freud developmental theory of psychoneuroses.[2]

• Oedipus complex is a universal experience in children and becomes a conscious desire in children between the age of 3-5 years old.
• Around the time of Stage IV, children begin to repress these feelings. The unconscious ego begins to develop and utilizes defense mechanisms to prevent the experience of these conflicting feelings. This leads to neurotic behaviors, perplexes, and disguised dream imagery.
• Psychoanalysis aimed to eliminate these neuroses

The first basic tenet of Freud's developmental theory found opposition early on. The idea that was held by Freud and his followers was that all psychoneuroses were a result of sexual trauma. One researcher, in particular, Charcot, was able to demonstrate the regular frequency with which psychoneuroses were developed with no indication of trauma. This runs directly counter to the Freudian assertion that childhood trauma and Oedipal phenomena are necessary for the development of psychoneuroses.[2] 

One of the more common criticisms of Freud's developmental theories, especially those regarding the Oedipus complex, is that many researchers feel that his theories are largely based on experimenter bias. Friedman and Downey discussed this likelihood in their paper, “Biology and the Oedipus Complex,” where they explore the likelihood that what Freud thought was Oedipal complex is just an evolutionary-based “play behavior” that manifests through competition.[3] The assertion is that the real source of the Oedipus complex is from Freud’s own experiences rather than those of the patients.

Despite these areas of concern, psychoanalysis has found itself influential in contemporary mental health care. It has abandoned part of its original basis in Freud's psychosexual developmental theory as well as the practice of the therapist’s interpretation of free association.

One must first discuss Freud's developmental theories on conscious and unconscious thinking, the id and ego, or as it is better known, the "structural theory of mind." The clinical significance of psychosexual development is found when we tie in this concept with the psychoanalytic drive theory and the structural theory. 

The id is simply the unconscious mind. It is by nature, inaccessible. It is essentially the biological, instinctual, unconscious drive of the individual as it is primarily involved in gratification. It is also where the repressed feelings and experiences of the individual are found. It has no direct interaction with the outside world. This is where the ego comes in, as it is the aspect of our personality that mediates the interaction between the id and the "real" world.

The ego is considered to be the conscious part of our personality. It is the part that constitutes things we "know" and how we think about and organize information and conscious experiences. It is rational by nature, whereas the id is irrational.[4][5]

Together, the id and the ego constitute the person, the drive constitutes where the conflict can arise, and psychosexual development as the mechanism through which various traumas present themselves as different neuroses. Much of early psychoanalysis was aimed at this interaction. The ego was identified as an integral part of the therapeutic process due to its role in mediation and conscious existence. The aim of psychoanalysis is to uncover many of these repressed aspects of the unconscious through understanding the etiology of a patient's pathological drives. Then the therapist works with the patient to manage these drives via the ego.[4][6]

Freud's contribution to the field of psychiatry has been lauded as one that expanded the scope in an unprecedented manner. Freud was able to bring some unification of the treatment of neuroses, a field of concern to neurologists, and the origin of these neuroses, a field predominantly of interest to psychologists. His focus was on the exploration of the unconscious mind through free association and how a practitioner goes about bringing that mind into a patient's consciousness. In doing this, Freud formed the basis for a large portion of contemporary psychotherapy.[7] Freud's contributions have spread far beyond just the realm of psychiatry into general medical practice as well as the humanities.[8]

Freud's developmental theories form the basis for psychoanalysis, and psychoanalysis has gone on to form the basis of a broad spectrum of modern talk therapies. Several concepts developed by psychoanalysis contributed to form the basis of psychodynamic psychotherapy. These core concepts include the importance of the therapeutic alliance, shared goals in therapy, and active listening by the therapist. All of which are present in any successful psychotherapeutic approach.[9]

One example of how dysfunction in Freud psychosexual development can influence patient care is discussed in a 2008 paper that investigates the change of shift handover ritual between psychiatric nurses. The handover from one nursing shift to another fits all the criteria as a ritual as defined by psychoanalytic theory. This ritual is influenced by the nurse's anxieties and their resultant perception of the patient. It is suggested that the handover acts as a ritual through which the nurses can keep their anxieties manageable and continue with the ongoing practice of caring for the patient.[10] [Level 5]