Attachment theory describes the dynamics of long-term relationships between humans. Its most important tenet is that an infant needs to develop a relationship with at least one primary caregiver for social and emotional development to occur normally. Attachment theory is an interdisciplinary study encompassing the fields of psychological, evolutionary, and ethological theory. Immediately after World War II, homeless and orphaned children presented many difficulties, and psychiatrist and psychoanalyst John Bowlby was asked by the United Nations (UN) to write a pamphlet on the matter. Later he went on to formulate attachment theory.

Infants become attached to adults who are sensitive and responsive in social interactions with them, and who remain as consistent caregivers for some months during the period from about six months to two years of age. When an infant begins to crawl and walk they begin to use attachment figures (familiar people) as a secure base to explore from and return to. Parental responses lead to the development of patterns of attachment; these, in turn, lead to internal working models which will guide the individual's perceptions, emotions, thoughts and expectations in later relationships. Separation anxiety or grief following the loss of an attachment figure is considered to be a normal and adaptive response for an attached infant. These behaviours may have evolved because they increase the probability of survival of the child.

Edward John Mostyn "John" Bowlby (26 February 1907 – 2 September 1990) was a British psychologist, psychiatrist and psychoanalyst, notable for his interest in child development and for his pioneering work in attachment theory.

Bowlby was born in London to an upper-middle-class family. He was the fourth of six children and was brought up by a nanny in the British fashion of his class at that time. His father, Sir Anthony Alfred Bowlby, first Baronet, was surgeon to the King's Household, with a tragic history: at age five, Sir Anthony's own father, Thomas William Bowlby, (John's grandfather) was killed while serving as a war correspondent in the Opium Wars^{[disambiguation needed ]}.

Normally, Bowlby saw his mother only one hour a day after teatime, though during the summer she was more available. Like many other mothers of her social class, she considered that parental attention and affection would lead to dangerous spoiling of the children. Bowlby was lucky in that the nanny in his family was present throughout his childhood. When Bowlby was almost four years old, his beloved nanny, who was actually his primary caretaker in his early years, left the family. Later, he was to describe this as tragic as the loss of a mother.

Mary Dinsmore Salter Ainsworth (December 1, 1913 – March 21, 1999) was a Canadian developmental psychologist known for her work in early emotional attachment with "The Strange Situation" as well as her work in the development of Attachment Theory.

Ainsworth was born in Glendale, Ohio in 1913, eldest of three sisters. parents both graduated from Dickinson College. Her father earned his Master's in History and was transferred to a manufacturing firm in Canada when Ainsworth was five. While her parents always put a strong emphasis on education, it was William McDougall's book Character and the Conduct of Life that inspired her interest in psychology.

Ainsworth enrolled in honors program in psychology at the University of Toronto in the fall of 1929. She earned her B.A. in 1935, her M.A. in 1936, and her Ph.D in 1939, all from the University of Toronto. She stayed to teach for a few years before joining the Canadian Women's Army Corp in 1942 in World War II, reaching the rank of Major in 1945.