An initial comparative study conducted on Mahler’s theory of separation-individualization, Stern’s layered model of development, and Bowlby’s attachment theory reckons that connection and separation are two sides of psychological development in mankind; the development of one side will bolster the development of the other side. Therefore, the connection from Stern’s theory and the separation from Mahler’s can complement and reinforce each other. They are equal in the utility of psychological development. The attachment theory focuses on attachment and connection, but secure attachment could promote development in both separation and individualization. Through the discussion on clinical cases, it is shown that clinical applications of Stern’s theory are relatively limited, but the theory of separation-individualization and the attachment theory to a great extent enhance the explanatory power of psychoanalysis, that the clinical applications are yet to be studied further.