Melanie Klein's contribution to psycho-analysis is centered on her exploration into the early stages of the mental development of the child, which she began while a pupil of Karl Abraham. Taking as her starting point Freud's concept of free association, as used in the analysis of adults, she set herself the task of adapting this technique to the psycho-analytic treatment of children. From this was evolved her play technique whereby, though providing a situation in which a child could play "freely", she was able to interpret his play - that is, describe and explain to him the feelings and phantasies that seemed to be expressed within it.By means of this technique, Klein made a most significant contribution to psycho-analysis. Not only she showed that it is possible to achieve therapeutic results more fruitful than those usually achieved with adults; but she was also able to map out in greater detail than had ever before been possible the early stages of mental development.The psycho-analytic picture as illuminated by her work emphasizes two principal stages of normal development, which Klein called "positions". The first, issuing from the infant's unintegrated and violently conflicting attitude to the vital objects of this world, is marked by a persecutory anxiety which may retard or disrupt the integration of the infant's ego. In the second stage the infant begins to apprehend that the gratifying objects he needs and loves are but other aspects of the menacing and frustrating objects he hates. This discovery arouses concern for these objects, and he experiences depression. However, in so far as he can tolerate the depressive position, it gives rise to reparative impulses and a capacity for unselfish concern and protective love. The extent to which he achieves this normal outcome determines the stability of his health or his liability to illness.

In Klein's view, two of Freud's great discoveries, the super-ego and the Oedipus complex, have their roots in these early periods of development.This is a milestone in Kleinian psycho-analysis, and includes contributions from among others Joan Riviere and W.R. Bion.