This research paper firstly provides the reader with a list of definition of related medical words, that were used in the paper. The researcher also presents a literature review that were conducted to determine the way that Melanie Klein’s psychoanalytic theories had changed our understanding concerning the infancy. As part of going through the main discussion, the researcher states that the way we have previously viewed infancy prior to Klein’s introduction of her psychoanalytic theories, specifically with Freud’s theories of infancy which includes ego/superego, ideas of phantasy, guilt, and anxiety were analyzed and discussed. This provides the readers with a better understanding regarding the impact on how Klein’s theories had molded the current theories behind infancy. The gap in Freud’s psychoanalytic theories with regards to enabling us to understand matters related to the emotional and mental well-being of the infants was narrowed down by Klein’s theories related to object-relations, play technique, views on envy and greed, and phantasy enabled us to have a better understanding about the uncovered developmental stages that the infants are going through. The researcher then concluds that the study behind the existing psychoanalytic theories behind the human development is a continuous process. Given that a new theorist can develop clinical evidences over a particular concept, there is a strong possibility that a new theory can foreshadow the past theories that we have once considered the best.