This article celebrates Ed Tronick’s contribution to psychodynamic developmental psychology. Tronick’s ideas have implicitly and explicitly influenced late 20th-century and early 21st-century psychoanalytic thinking, and his contribution is here acknowledged in relation to the work on the developmental ideas concerning mentalizing. Tronick’s (1989, 2007) mutual regulation model (MRM) is outlined and examined from the point of view of the attachment-theory-based mentalization model of interpersonal interaction and psychopathology. The article identifies common ground between clinical implications of Tronick’s MRM and the mentalization model as independent but complementary ways of expanding ideas originating in attachment theory, and highlights ways in which the MRM provides greater clarity for several concepts which are frequently used in writings about mentalizing such as the background of safety and the holding environment.