Melanie Klein (b. 1882–d. 1960) proposed a revolutionary way of thinking about children and child psychoanalysis that led to discoveries related to the understanding of the functioning, structure, and growth of the mind, as well as an original method for the psychoanalytic treatment of children. The creation of her play technique redefined child psychoanalysis, transformed adult psychoanalysis, and opened new areas including the psychoanalysis of psychosis, autism and borderline conditions, group analysis, and interdisciplinary studies on issues that affect the child’s world. Klein started working with children at a time when child analysis occupied a marginal position in psychoanalysis; children were considered unanalyzable, or in danger if analyzed. Klein’s freedom of thought led her to test psychoanalytic theory and method in her clinical encounters with children, resulting in her concept of the child as a unique object of psychoanalytic treatment and investigation. Her radically original approach to child analysis facilitated the study and treatment of the earliest and deepest functioning of the psyche. The infant was conceived of as born with its objects and with an ego from birth, equipping the infant with the capacity to relate, to love, and to hate the other, to differentiate between me and not-me, inside and outside, and to phantasize. Klein’s theory of the paranoid-schizoid and depressive positions is a theory on changes in the link between the ego and its objects and ensuing anxieties. The task in development is to master this related anxiety and transform it into language and thought, this occurring in the context of the relationship with the mother/other/analyst. Klein took children extremely seriously. Her child clinical material offers vivid descriptions of the child’s mind and the contact she made with challenging young patients. Klein had an obvious passion for clinical work and curiosity about the child’s unconscious discourse, her intuition, and capacity for observation. Available for the child, playing out the roles attributed to her, adhering to phantasy, receiving positive and negative transferences, intuitively registering anxiety and interpreting it: Klein gives meaning to the child’s experience. The main focus of this article is on the relevance of Kleinian discourse for the study of the child and the thinking on the needs of and dangers affecting children in the 21st century.