The purpose of this thesis [Swedish language with English abstract and summary] was to study the reactions of infants when they started attending day care centers and to elucidate the process of adjustment. The theoretical perspective employed is the J. Bowlby/M. Ainsworth attachment theory. U. Bronfenbrenner's ecological theory of human development has been used as a complement. A model was developed to analyze infants' responses to maternal separation and short-term effects of early entry into day care. A sample of 26 day-care infants (ages 6-12 months when entering day care) and a matched sample of 26 infants (cared for exclusively by their mothers) were studied during a period of 5 months. The data focused on repeated observations of infant-mother interactions at home and also on infant-staff interactions (for the day care sample) at 18 different day-care centers. Comparisons on a group level demonstrated adjustment in two phases. The first phase, called the initial period, lasted between 3 and 4 weeks and was marked with sadness and mild distress. The second phase, called the period of adaptation, beginning after about 7 weeks of day-care attendance and onwards, showed a marked change of adjustment. Results demonstrated that a majority of the day care infants reacted with sadness and mild distress. Following the period of adaptation, adjustment to the center setting was characterized by extremely low levels of crying and a low level of infant-staff interaction. Quality of day care and quality of mother-infant interactions were important factors influencing the infants' more immediate reactions to day care and the nature of the process of adjustment to the center setting. (Contains over 300 references.) (Author/WJC)