In this dissertation the association of the quality of early mother-infant interaction with the child’s physical health during the subsequent two years (n=120, Study I) and with the child’s behavioural and emotional symptoms at two years (n=50, Study II) was examined. Maternal psychopathology (Studies II and IV) as well as the mother’s close relationships (n=131, Study IV) were investigated as possible determinants of the mother’s behaviour in early mother-infant interaction with her infant. The association between the quality of early mother-infant interaction and mother’s perception of her child’s temperament was investigated (n=124, Study III). Also, the role of the infant in early mother-infant interaction was addressed (Studies I-III), and aspects of the dyad were considered. Studies I and II were longitudinal and Studies III and IV cross-sectional. The participants were from the Finnish subsample of a more extensive European early intervention study (the European Early Promotion Project, EEPP). The mothers were interviewed when the infants were 4-10 weeks of age, and as a part of the interview, the mothers’ mental health was assessed using a structured diagnostic psychiatric interview (Structured Clinical Interview for DSM-IV, SCID). The mothers completed questionnaires on parenting stress (Parenting Stress Index, PSI) and their infant’s temperament (Infant Characteristics Questionnaire, ICQ). Mother-infant interaction was videotaped when the infants were 8-11 weeks of age, and analysed using Global Rating Scale for Mother-Infant Interaction (GRS). After two years a follow-up interview was conducted and the mothers completed a questionnaire on their child’s behaviour (Child Behavior Checklist, CBCL). Certain deviant features in early mother-infant interaction were associated with the child’s poorer outcome. Infant’s poor interactive behaviour (i.e. avoidance and a lack of active communication and positive vocalisations) predicted the child’s chronic or recurrent health problems during the subsequent two years. Mother’s hostility and/or intrusiveness was a significant predictor of high externalising symptom scores and high total problem scores of the child at two years assessed by CBCL. Surprisingly, mother’s mental health problems were not related to maternal behaviour in interaction with her two-month-old infant whereas the mother’s childhood relationship with her own mother was an important determinant of her behaviour. Also, among mothers with mental health problems poor/disengaged marital relationship was associated with the mother’s poorer interactive behaviour. Infant’s poor interactive behaviour and mother’s intrusiveness in early mother-infant interaction increased the infant’s risk of being perceived as difficult. The role of the infant as an active partner of the dyad was already important when the infant was two months old. The results of this study imply that an important determinant of infant outcome may be the mutual adaptation capacity of the dyad - the extent to which the dyad, the infant and the mother, can adjust to achieve a balance between the two of them, but also in relation to complex systems of other relationships in the family, and in relation to other internal or external states. The deviant features that have been shown to relate to the infant’s less than optimal outcome may be seen as indicators of the overloaded mutual adaptation capacity of the dyad. Better and more accurate observational methods are still needed in order to be able to assess the infant’s behaviour, and the functioning of the dyad in more detail. In the future, when assessing infants and early mother-infant interaction we need to combine these two aspects: a detailed observation and a systemic view of the dyad.