Thesis to opt to the degree of doctor in psychotherapy / The prevalence of social-emotional problems in early childhood continues at a high level (Centro de Microdatos-Universidad de Chile, 2014). This stage is a critical period in which the immediate family is the most influential system in childhood development (Bronfenbrenner, 1987). Conversely, the parental reflective function (RF) is considered a protective factor in early parenting (Stacks et al., 2014), assuming a relevant role in socialemotional development in early childhood (Ensink, Bégin, Normandin, & Fonagy, 2016; Smaling, Huijbregts, van der Heijden, van Goozen, & Swaab, 2016a). Objective: To describe and analyze the relationship between fathers’ and mothers’ RFs, the quality of the mother-father-child triadic interaction, and children’s psychomotor development and social-emotional difficulties. Method: A non-experimental, transversal and correlational study was developed. Fifty mother-father-child triads, each in a current relationship that included at least one child from 12–36 months of age, were evaluated. Sociodemographic background, triadic interaction (LTP, Fivaz-Depeursingue & Corboz-Warnery, 1999), parental RF (PDI-S, Slade, Aber, Berger, Bresgi, & Kaplan, 2012, assessed by RF Scales, Fonagy, Steele, Steele, & Target, 1998), psychomotor development (ASQ-3, Squires & Bricker, 2009) and social-emotional difficulties (ASQ SE, Squires, Bricker, & Twombly, 2002) were measured. Couple relationship satisfaction (RAS, Hendrick, 1988) and depressive symptoms in the parents (BDI-I, Beck, Ward, Mendelson, Mock & Erbaugh, 1961) were included as control variables. Results: A significant effect of the triadic interaction on the child’s social-emotional difficulties was found. The effect explained 20% of the variance. However, this effect was not found in the psychomotor development. In addition, the mothers’ RF had a significant influence on the triadic interaction, explaining 21% of the variance. However, in contrast to the hypothesis, the mothers’ and fathers’ RFs were not significant variables as direct or indirect predictors to explain the child’s socio-emotional difficulties or psychomotor development. These findings show the importance of the RF on the quality of the mother-fatherchild interaction, which in turn influences the child’s social-emotional development. Additionally, the role of the father and the implications of these findings for research and clinical purposes are discussed.